drm/amd/pm: add pptable_funcs documentation (v3)

Documents the hooks in struct pptable_funcs.

v2: Improved documentation accuracy.
v3: Improved set_default_od_settings() definition.
Signed-off-by: Ryan Taylor <Ryan.Taylor@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>