drm/amd/display: Delete unimplemented dm_pp_apply_power_level_change_request() (v2)

drm/amd/display: Delete unimplemented dm_pp_apply_power_level_change_request() (v2)

dm_pp_apply_power_level_change_request() was called from old
DCE clock manager implementations on DCE6, 8, 10, 11.2
but has not been implemented ever since the beginning of DC.

Affected GPUs have been working fine without that implementation
for many years. Let's delete it now.

v2:
- Delete dm_pp_apply_power_level_change_request too

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>