drm/amd/amdgpu: ring debugfs is read in increments of 4 bytes

If a user tries to read a non-multiple of 4 bytes it would have
read until the end of the ring potentially crashing the user
task.

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