drm/radeon/kms: Return to userspace on ERESTARTSYS

radeon_object.h wasn't converted to ERESTARTSYS change. No
each time we got an ERESTARTSYS we return to userspace (ie
we were interrupted by a signal and we let the userspace
reschedule the ioctl).

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>