drm/amd/powerplay/fiji: mark symbols static where possible

drm/amd/powerplay/fiji: mark symbols static where possible

We get a few warnings when building kernel with W=1:
drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/fiji_hwmgr.c:1633:9: warning: no previous prototype
for 'fiji_get_voltage_index' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/fiji_hwmgr.c:5412:5: warning: no previous prototype
for 'fiji_check_states_equal' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/fiji_thermal.c:424:5: warning: no previous prototype
for 'tf_fiji_thermal_setup_fan_table' [-Wmissing-prototypes]
....

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
So this patch marks these functions with 'static'.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>