glamor: Fix debugging callback setup on GLES

You will not find GL_ARB_* extensions in a GLES context by definition,
the droid you're looking for is named GL_KHR_debug.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
4 jobs for glamor-gles-debugging in 7 minutes and 56 seconds (queued for 7 minutes and 31 seconds)