drm/amdgpu: limit the number of IVs processed at once

Only process a maximum of 32 IVs before writing back the RPTR. This improves
hw handling when we get close to an overflow in the ring buffer.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>