drm/amd/include/vega20_ip_offset: Mark top-level IP_BASE definition as __maybe_unused

drm/amd/include/vega20_ip_offset: Mark top-level IP_BASE definition as __maybe_unused

 In file included from drivers/gpu/drm/amd/amdgpu/vega20_reg_init.c:27:
 drivers/gpu/drm/amd/amdgpu/../include/vega20_ip_offset.h:154:29: warning: ‘XDMA_BASE’ defined but not used [-Wunused-const-variable=
 154 | static const struct IP_BASE XDMA_BASE ={ { { { 0x00003400, 0, 0, 0, 0, 0 } },
 | ^~~~~~~~~
 drivers/gpu/drm/amd/amdgpu/../include/vega20_ip_offset.h:63:29: warning: ‘FUSE_BASE’ defined but not used [-Wunused-const-variable=]
 63 | static const struct IP_BASE FUSE_BASE ={ { { { 0x00017400, 0, 0, 0, 0, 0 } },
 | ^~~~~~~~~

Fixes the following W=1 kernel build warning(s):

Acked-by: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>