drm/amd/powerplay: mark symbols static where possible

drm/amd/powerplay: mark symbols static where possible

We get a few warnings when building kernel with W=1:
drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/fiji_smumgr.c:162:5: warning: no previous prototype for 'fiji_setup_pwr_virus' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/fiji_smc.c:2052:5: warning: no previous prototype for 'fiji_program_mem_timing_parameters' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/polaris10_smumgr.c:175:5: warning: no previous prototype for 'polaris10_avfs_event_mgr' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/cz_hwmgr.c:69:10: warning: no previous prototype for 'cz_get_eclk_level' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/smu7_hwmgr.c:92:26: warning: no previous prototype for 'cast_phw_smu7_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'.

Acked-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>