st/dri: enable EGL_ANDROID_blob_cache on gallium drivers

Verified to work properly with Iris driver on Android Celadon. Cache
files get generated as 'com.android.opengl.shaders_cache' for each
application.

v2: check that cache was returned (Eric Engestrom)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
10 jobs for !939 with EGL_ANDROID_blob_cache-for-gallium in 10 minutes and 19 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed #314947
debian

00:00:17

 
  Build+Test
passed #314949
meson-clang

00:06:56

passed #314953
meson-clover

00:10:02

passed #314952
meson-main

00:07:33

passed #314948
meson-swr-glvnd

00:07:46

passed #314954
meson-vulkan

00:03:19

passed #314956
scons-llvm

00:07:15

passed #314955
scons-nollvm

00:03:01

passed #314950
scons-swr

00:05:02

passed #314951
scons-win64

00:09:50