drm/radeon: fix correctness of irq_enabled check for radeon.

This check was introduced with the logic the wrong way around.

Fixes regression: http://bugzilla.kernel.org/show_bug.cgi?id=12216

Tested-by: François Valenduc <francois.valenduc@tvcablenet.be>
Signed-off-by: Dave Airlie <airlied@redhat.com>