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pan/mdg: Limit work registers for large workgroups

Alyssa Rosenzweig requested to merge alyssa/mesa:midgard/thread-group into main 
When more than 8 registers are used, Midgard can only fit 64 threads in a
thread group. For barriers to work properly, a threadgroup must fit an entire
work group. The GL driver configures the hardware to have threadgroups the size
of work groups. That means if more than 64 threads are used in a workgroup, and
more than 8 registers are used, the hardware will fault spawning threads.

To workaround this hardware limitation, we need to limit the number of work
registers used depending on the size of the workgroup. Typically, the work group
size is known at compile-time so that determination can usually be made without
variants. To avoid variants, we make a pessimistic estimate in the case when
it's not known at compile-time.

shader-db shows 6 shaders affected. I expect that all of these would fault with
DATA_INVALID_FAULT if they tried to execute before this patch, due to the
oversize local size, and faulting is even slower than spilling ;-)

Fixes dEQP-GLES31.functional.synchronization.* on Mali-T860.

Closes: #7228 (closed)

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