drm/amdgpu: allow framebuffer in GART memory as well

On CZ and newer APUs we can pin the fb into GART as well as VRAM.

v2: Don't enable gpu_vm_support for Raven yet since it leads to
    a black screen. Need to debug this further before enabling.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Samuel Li <samuel.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>