iris: Advertise coherent framebuffer fetches

This lets us advertise GL_EXT_shader_framebuffer_fetch and
GL_KHR_blend_equation_advanced_coherent support.
10 jobs for !732 with iris-fbfetch in 8 minutes and 32 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #300383
debian

00:00:22

 
  Build+Test
passed #300385
meson-clang

00:07:40

passed #300389
meson-clover

00:08:09

passed #300388
meson-main

00:05:56

passed #300384
meson-swr-glvnd

00:04:41

passed #300390
meson-vulkan

00:02:56

passed #300392
scons-llvm

00:04:21

passed #300391
scons-nollvm

00:03:25

passed #300386
scons-swr

00:02:48

passed #300387
scons-win64

00:05:50