drm/amdgpu: enable GTT fallback handling for dGPUs only

That is just a waste of time on APUs.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3704


Fixes: 216c1282 ("drm/amdgpu: use GTT only as fallback for VRAM|GTT")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit e8fc090d)
Cc: stable@vger.kernel.org