Without this, reg_offset will return 1024 for acc0. This causes
has_invalid_dst_region to decide that the destination region is invalid
(because 1024 != 0), and the lowering code tries to treat the floating
point accumulators as integers. It's a mess.
v2: Add and use set_gfx_platform. Suggested by Caio.
Fixes: 937373eb25 ("i965/fs: Handle fixed HW GRF subnr in reg_offset().")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40716>
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