drm/amd/powerplay:clean up phm_enable_clock_power_gatings

As the PG was setted by each IP block durinng IP early init thus
remove the unused phm_enable_clock_power_gatings related funcs.

Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>