drm/amdgpu: further mitigate workaround for i915

Disable the workaround on imported BOs as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexdeucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>