egl: rework handling EGL_CONTEXT_FLAGS

egl: rework handling EGL_CONTEXT_FLAGS

As of version 15 of the EGL_KHR_create_context spec, debug contexts
are allowed for ES contexts.  We should allow creation instead of
erroring.

While we're here provide a more comprehensive checking for the other two
flags - ROBUST_ACCESS_BIT_KHR and FORWARD_COMPATIBLE_BIT_KHR

v2 [Emil Velikov] Rebase. Minor tweak in commit message.

Cc: Boyan Ding <boyan.j.ding@gmail.com>
Cc: Chad Versace <chad.versace@intel.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91044
Signed-off-by: Matthew Waters <ystreet00@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>