drm/amdgpu/display: set gpu vm flag for all asics which support it

It won't get used unless the driver allows the gtt domain for
display buffers which is controlled elsewhere.

Acked-by: Huang Rui <ray.huang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>