drm/amdgpu: add selftest framework for UMSCH

Prepare for VPE and VCN queue submission test.

v2: rebase on drm_exec (Alex)
Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Veerabadhran Gopalakrishnan <Veerabadhran.Gopalakrishnan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>