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 arm_build #1444621
aarch64

00:00:21

passed arm_test #1444622
aarch64

00:00:22

passed x86_build #1444618

00:00:19

passed x86_build_old #1444620

00:00:19

passed x86_test-gl #1444619

00:00:26

 
  Build
passed meson-arm64 #1444626
aarch64

00:11:42

passed meson-armhf #1444625
aarch64

00:05:00

passed meson-clang #1444627

00:09:30

passed meson-clover #1444630

00:04:44

passed meson-clover-old-llvm #1444631

00:06:04

passed meson-i386 #1444633

00:04:12

passed meson-main #1444624

00:10:38

passed meson-mingw32-x86_64 #1444634

00:04:16

passed meson-testing #1444623

00:07:44

passed meson-vulkan #1444632

00:05:11

passed scons #1444635

00:06:03

passed scons-old-llvm #1444636

00:03:38

passed scons-swr #1444628

00:10:40

passed scons-win64 #1444629

00:11:53

 
  Test
passed arm64_a306_gles2 #1444655
db410c

00:07:06

passed arm64_a630_gles2 #1444652
mesa-cheza

00:02:11

passed arm64_a630_gles3 #1444654
mesa-cheza

00:06:30

passed arm64_a630_gles31 #1444653
mesa-cheza

00:04:37

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

00:06:29

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

00:08:14

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

00:05:24

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

00:06:23

passed piglit-glslparser #1444642

00:05:04

passed piglit-quick_gl #1444641

00:09:23

passed piglit-quick_shader #1444643

00:05:55

passed test-llvmpipe-gles2 #1444644

00:03:01

passed test-softpipe-gles2 #1444645

00:01:15

passed test-softpipe-gles31 1/4 #1444648

00:02:52

passed test-softpipe-gles3 1/2 #1444646

00:05:03

passed test-softpipe-gles31 2/4 #1444649

00:04:07

passed test-softpipe-gles31 3/4 #1444650

00:05:00

passed test-softpipe-gles31 4/4 #1444651

00:03:21

passed test-softpipe-gles3 2/2 #1444647

00:03:54