drm/amdgpu: move kiq_reg_write_reg_wait() out of amdgpu_virt.c

It's used for more than just SR-IOV now, so move it to
amdgpu_gmc.c and rename it to better match the functionality and
update the comments in the code paths to better document
when each path is used and why.  No functional change.

Reviewed-by: Shaoyun.liu <Shaoyun.liu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Shaoyun.Liu@amd.com
Cc: Christian.Koenig@amd.com