drm/amdgpu: fix VCE buffer placement restrictions v2

Turned out that VCE still has a placement restriction that BOs can't
cross a 4GB boundary.

Fix this by adding a command submission parser prepass to correctly
place the buffers.

v2: add function description

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