i965: Fix out-of-bounds access to brw_stage_state::surf_offset

../src/mesa/drivers/dri/i965/brw_wm_surface_state.c:1378:32: runtime error: index 3503345872 out of bounds for type 'uint32_t [149]'

brw_assign_common_binding_table_offsets has the following comment:

 "Unused groups are initialized to 0xd0d0d0d0 to make it obvious that they're
 unused but also make sure that addition of small offsets to them will
 trigger some of our asserts that surface indices are < BRW_MAX_SURFACES."

Cc: 20.0 mesa-stable@lists.freedesktop.org

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