drm/radeon: fix irq ring buffer overflow handling

commit e8c214d22e76dd0ead38f97f8d2dc09aac70d651 upstream.

We must mask out the overflow bit as well, otherwise
the wptr will never match the rptr again and the interrupt
handler will loop forever.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
[bwh: Backported to 3.2: drop changes for unsupported GPUs]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>