drm/amdgpu: only support IBs in the buffer list (v2)

amdgpu_cs_find_mapping doesn't work without all buffers being validated,
so the TTM validation must be done first.

v2: only use amdgpu_cs_find_mapping for UVD/VCE VM emulation

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>