meson: Prefer 'iris' by default over 'i965'.

This changes the default driver for Intel Gen8-11 hardware to be
the newer 'iris' driver rather than the older 'i965' driver.  To
continue using i965, pass -Dprefer-iris=false when building.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Marge Bot <mesa/mesa!3540>
Part-of: <mesa/mesa!3540>
38 jobs for !3540 with iris-default in 20 minutes and 24 seconds (queued for 3 seconds)
merge request