iris: add support for INTEL_conservative_rasterization

this hooks up the iris gallium driver to existing mesa bits which handle the implementation

resolves kwg/mesa#8
14 jobs for !681 with intel-conservativeraster in 13 minutes and 58 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #254744
debian

00:00:21

 
  Build+Test
passed #254746
meson-clang

00:13:37

passed #254752
meson-gallium-clover-llvm

00:03:44

passed #254753
meson-gallium-clover-llvm39

00:06:58

passed #254751
meson-gallium-drivers-other

00:01:43

passed #254750
meson-gallium-radeonsi

00:06:23

passed #254754
meson-gallium-st-other

00:07:20

passed #254745
meson-gallium-swr

00:10:50

passed #254749
meson-glvnd

00:01:43

passed #254748
meson-loader-classic-dri

00:01:44

passed #254747
meson-vulkan

00:05:04

passed #254756
scons-llvm

00:03:36

passed #254755
scons-nollvm

00:04:37

passed #254757
scons-swr

00:09:14