drm/radeon: Unreference GEM object outside of spinlock in page flip error path.

Should fix https://bugzilla.redhat.com/show_bug.cgi?id=726277

 .

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>