drm/radeon: Clear RB_OVERFLOW bit earlier

Otherwise the bit remains set in rdev->ih.rptr, so the wptr can never
match that and we still have an infinite loop.

This fix allows me to successfully recover from an IH ring buffer
overflow.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>