iris: fix register spilling on compute shaders on XeHP

DG2 scratch space is handled differently. Commit ae18e1e7 implemented support for it, but handled it differently between render and compute shaders: it calculates scratch_addr differently and doesn't pin the buffer on compute. Make it work on compute shaders by calling pin_scratch_space() from iris_compute_walker(), which fixes both the address and the pinning.

This commit can be verified by the two-year-old-but-still-unreviewed Piglit MR 234. You can also verify this by running a very simple compute shader with INTEL_DEBUG=spill_fs.

References: piglit!234 Fixes: ae18e1e7 ("iris: Add support for scratch on XeHP") Signed-off-by: Paulo Zanoni