drm/amdgpu: Use kvmalloc for allocating UVD/VCE/VCN BO backup memory

The allocated size can be (at least?) as large as megabytes, and
there's no need for it to be physically contiguous.

May avoid spurious failures to initialize / suspend the corresponding
block while there's memory pressure.

Bugzilla: https://bugs.freedesktop.org/107432


Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>