drm/amd/amdgpu: Mask rptr as well in ring debugfs

The read/write pointers on sdma4 devices increment
beyond the ring size and should be masked.  Tested
on my Ryzen 2400G.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>