[Why]
Scatter/gather feature is supported on Vangogh.

[How]
Add GTT domain support for Vangogh to enable
display buffers in system memory.
Signed-off-by: Roman Li <roman.li@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>