drm/amdgpu: return IH ring drain finished if ring is empty

Sometimes IH do not setup ring wptr overflow flag after wptr exceed
rptr. As a workaround, if IH rptr equals to wptr, ring is empty,
return true to indicate IH ring checkpoint is processed, IH ring drain
is finished.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>