drm/amd/powerplay/polaris10: mark symbols static where possible

drm/amd/powerplay/polaris10: mark symbols static where possible

We get a few warnings when building kernel with W=1:
drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/polaris10_clockpowergating.c:34:5: warning: no previous prototype
for 'polaris10_phm_powerup_uvd' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/polaris10_thermal.c:428:5: warning: no previous prototype
for 'tf_polaris10_thermal_setup_fan_table' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/polaris10_hwmgr.c:125:31: warning: no previous prototype
for 'cast_phw_polaris10_power_state' [-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: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>