drm/radeon/si_dpm: Replace one-element array with flexible-array in struct SISLANDS_S...

drm/radeon/si_dpm: Replace one-element array with flexible-array in struct SISLANDS_SMC_SWSTATE

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of a flexible-array member in
struct SISLANDS_SMC_SWSTATE, instead of a one-element array, and use
the struct_size() helper to calculate the size for the allocation.

Also, this helps with the ongoing efforts to enable -Warray-bounds by
fixing the following warnings:

drivers/gpu/drm/radeon/si_dpm.c: In function ‘si_convert_power_state_to_smc’:
drivers/gpu/drm/radeon/si_dpm.c:2350:20: warning: array subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’ {aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
 2350 |   smc_state->levels[i].dpm2.MaxPS = (u8)((SISLANDS_DPM2_MAX_PULSE_SKIP * (max_sclk - min_sclk)) / max_sclk);
      |   ~~~~~~~~~~~~~~~~~^~~
drivers/gpu/drm/radeon/si_dpm.c:2351:20: warning: array subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’ {aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
 2351 |   smc_state->levels[i].dpm2.NearTDPDec = SISLANDS_DPM2_NEAR_TDP_DEC;
      |   ~~~~~~~~~~~~~~~~~^~~
drivers/gpu/drm/radeon/si_dpm.c:2352:20: warning: array subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’ {aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
 2352 |   smc_state->levels[i].dpm2.AboveSafeInc = SISLANDS_DPM2_ABOVE_SAFE_INC;
      |   ~~~~~~~~~~~~~~~~~^~~
drivers/gpu/drm/radeon/si_dpm.c:2353:20: warning: array subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’ {aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
 2353 |   smc_state->levels[i].dpm2.BelowSafeInc = SISLANDS_DPM2_BELOW_SAFE_INC;
      |   ~~~~~~~~~~~~~~~~~^~~
drivers/gpu/drm/radeon/si_dpm.c:2354:20: warning: array subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’ {aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
 2354 |   smc_state->levels[i].dpm2.PwrEfficiencyRatio = cpu_to_be16(pwr_efficiency_ratio);
      |   ~~~~~~~~~~~~~~~~~^~~
drivers/gpu/drm/radeon/si_dpm.c:5105:20: warning: array subscript 1 is above array bounds of ‘SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’ {aka ‘struct SISLANDS_SMC_HW_PERFORMANCE_LEVEL[1]’} [-Warray-bounds]
 5105 |   smc_state->levels[i + 1].aT = cpu_to_be32(a_t);
      |   ~~~~~~~~~~~~~~~~~^~~~~~~

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Build-tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/603f9a8f.aDLrpMFzzSApzVYQ%25lkp@intel.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>