drm/amd/display: Add GART memory support for dmcub

[Why]
In dump file, GART memory can be accessed while frame buffer cannot.

[How]
Add GART memory support for dmcub.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Fudongwang <fudong.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>