drm/amdgpu: Remove powerdown_uvd member

With phm_powerdown_uvd() gone in the previous patch, there's
now no longer anything that reads the powerdown_uvd member of the
pp_hwmgr_func.

Remove it.

There are a few assignments to it; a boring NULL which can just go,
and two functions, but those functions are called explicitly anyway
so the assignments to the member go.

One of those (smu7_powerdown_uvd) wasn't static previously;
make it static.

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