drm/radeon: fix missing bo reservation

commit 5e386b574cf7e1593e1296e5b0feea4108ed6ad8 upstream.

Otherwise we might get a crash here.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>