Add support for disabling the VRT (Variable Register Thread) feature.
The strategy here is to force the old BRW_MAX_GRF limit for the
register allocator (locks the upper limit) and make sure
ptl_register_blocks() always return that amount of blocks (locks
the lower limit).
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35781>
887642b0f2