drm/amd/pm: Remove redundant function members of pptable_funcs

Remove redundant functions members of pptable_funcs and change
the function type as static because they are not called by other
files.

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>