drm/amdgpu/gfx11: select HDP ref/mask according to gfx ring pipe

Use correct ref/mask for differnent gfx ring pipe. Ported from
ZhenGuo's patch for gfx10.

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