intel/urb: fix allocation on Gen8-10 GT1

Error reported with simulator.

v2: Unsure the push constants are occupying the space we cannot use.

v3: Just assert the push constants are at least 4 chunks (Jason)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
41 jobs for !3166 with review/gen8-9-gt1-urb-allocation-fix in 17 minutes and 41 seconds (queued for 14 seconds)
latest detached
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