freedreno/a6xx: Fix MSAA clear

We need to do MSAA clear on the 3d pipe, it seems to not work out properly on 2d pipe (at least for 4x MSAA and heights that are not multiple of 16).

This matches what blob and tu seem to do. Fixes the following with DEQP_CONFIG=rgba8888d24s8ms4

dEQP-GLES31.functional.primitive_bounding_box.depth.builtin_depth.per_primitive_bbox_equal dEQP-GLES31.functional.primitive_bounding_box.depth.builtin_depth.per_primitive_bbox_larger dEQP-GLES31.functional.primitive_bounding_box.depth.user_defined_depth.per_primitive_bbox_equal dEQP-GLES31.functional.primitive_bounding_box.depth.user_defined_depth.per_primitive_bbox_larger

Signed-off-by: Rob Clark robdclark@chromium.org