On gfx9 hardware the value is not wrapped and is a 64-bit value.  So
we reduce it modulo the ring size.
Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>

(v2) use buf_mask instead of computing on the fly
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>