drm/amdgpu: fix and cleanup UVD IB generation

We didn't synced the BO after validating it. Also sart to use
amdgpu_bo_create_reserved to simplify things.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>