drm/radeon/kms: r600/r700 don't process IRQ if not initialized

In some rare case the wptr returned from the hw wasn't 0 and leaded
to trick r600_process_irq that their were irq to process. Add a
check to bail out if irq hasn't been initialized this will avoid
oops provoqued by the rare wptr != 0 on initialization.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>