drm/radeon: never unpin UVD bo v3

Changing the UVD BOs offset on suspend/resume doesn't work because the VCPU
internally keeps pointers to it. Just keep it always pinned and save the
content manually.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=66425



v2: fix compiler warning
v3: fix CIK support

Note: a version of this patch needs to go to stable.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>