iris: Enable the dual_color_blend_by_location driconf option.

This fixes rendering in Unigine Valley 1.0 and Heaven 4.0.
14 jobs for !700 with iris-driconf in 8 minutes and 34 seconds (queued for 4 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #248998
debian

00:00:21

 
  Build+Test
passed #249000
meson-clang

00:07:34

passed #249006
meson-gallium-clover-llvm

00:04:57

passed #249007
meson-gallium-clover-llvm39

00:05:10

passed #249005
meson-gallium-drivers-other

00:08:11

passed #249004
meson-gallium-radeonsi

00:03:55

passed #249008
meson-gallium-st-other

00:04:30

passed #248999
meson-gallium-swr

00:07:47

passed #249003
meson-glvnd

00:06:06

passed #249002
meson-loader-classic-dri

00:04:57

passed #249001
meson-vulkan

00:04:27

passed #249010
scons-llvm

00:03:42

passed #249009
scons-nollvm

00:04:56

passed #249011
scons-swr

00:06:55