drm/amdgpu: Remove phm_powerdown_uvd

phm_powerdown_uvd() has been unused since 2017's
commit 47047263 ("drm/amd/powerplay: delete eventmgr related files.")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>