iris: Increase the maximum grid size

iris: Increase the maximum grid size

I had simply copied these values from another driver when adding initial
compute support to iris.  The actual hardware limit is UINT32_MAX (see
the GPGPU_WALKER/COMPUTE_WALKER ThreadGroupID{X,Y,Z}Dimension fields).

Thanks to Karol Herbst for noticing the unnecessarily low limit.

References: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7676
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19826>