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)
detached
Status Job ID Name Coverage
  Container
passed #1444621
aarch64
arm_build

00:00:21

passed #1444622
aarch64
arm_test

00:00:22

passed #1444618
x86_build

00:00:19

passed #1444620
x86_build_old

00:00:19

passed #1444619
x86_test-gl

00:00:26

 
  Build
passed #1444626
aarch64
meson-arm64

00:11:42

passed #1444625
aarch64
meson-armhf

00:05:00

passed #1444627
meson-clang

00:09:30

passed #1444630
meson-clover

00:04:44

passed #1444631
meson-clover-old-llvm

00:06:04

passed #1444633
meson-i386

00:04:12

passed #1444624
meson-main

00:10:38

passed #1444634
meson-mingw32-x86_64

00:04:16

passed #1444623
meson-testing

00:07:44

passed #1444632
meson-vulkan

00:05:11

passed #1444635
scons

00:06:03

passed #1444636
scons-old-llvm

00:03:38

passed #1444628
scons-swr

00:10:40

passed #1444629
scons-win64

00:11:53

 
  Test
passed #1444655
db410c
arm64_a306_gles2

00:07:06

passed #1444652
mesa-cheza
arm64_a630_gles2

00:02:11

passed #1444654
mesa-cheza
arm64_a630_gles3

00:06:30

passed #1444653
mesa-cheza
arm64_a630_gles31

00:04:37

passed #1444640
lava-meson-gxl-s905x-libretech-cc
lima-mali450-test:arm64

00:06:29

passed #1444637
lava-sun50i-h6-pine-h64
panfrost-t720-test:arm64

00:08:14

passed #1444638
lava-rk3288-veyron-jaq
panfrost-t760-test:armhf

00:05:24

passed #1444639
lava-rk3399-gru-kevin
panfrost-t860-test:arm64

00:06:23

passed #1444642
piglit-glslparser

00:05:04

passed #1444641
piglit-quick_gl

00:09:23

passed #1444643
piglit-quick_shader

00:05:55

passed #1444644
test-llvmpipe-gles2

00:03:01

passed #1444645
test-softpipe-gles2

00:01:15

passed #1444648
test-softpipe-gles31 1/4

00:02:52

passed #1444646
test-softpipe-gles3 1/2

00:05:03

passed #1444649
test-softpipe-gles31 2/4

00:04:07

passed #1444650
test-softpipe-gles31 3/4

00:05:00

passed #1444651
test-softpipe-gles31 4/4

00:03:21

passed #1444647
test-softpipe-gles3 2/2

00:03:54