drm/amdgpu: linear validate first then bind to GART

For VM emulation for old UVD/VCE we need to validate the BO with linear
VRAM flag set first and then eventually bind it to GART.

Validating with linear VRAM flag set can move the BO to GART making
UVD/VCE read/write from an unbound GART BO.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
CC: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>