drm/amdgpu: create amdgpu_vkms (v4)

Modify the VKMS driver into an api that dce_virtual can use to create
virtual displays that obey drm's atomic modesetting api.

v2: Made local functions static.

v3: Switched vkms_output kzalloc for kcalloc.
    Cleanup patches by moving display mode fixes to this patch.

v4: Update atomic_check and atomic_update to comply with new kms api.

Signed-off-by: Ryan Taylor <Ryan.Taylor@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>