drm/amdgpu: add VCE VM mode support

This adds VCE VM mode support from Stoney onwards. Session tracking
is an open issue, yet to be supported.

v2: Fixed warnings from checkpatch.pl

Signed-off-by: Maruthi Bayyavarapu <maruthi.bayyavarapu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>