egl: Quiet warning about front buffer rendering for pixmaps/pbuffers

pbuffer configs cause a million of these warnings to trigger, but
when using pixmaps or buffers, there is only one surface, so this
warning doesn't make much sense.  Retain it for window surfaces for now.

Fixes: dacb11a5 ("egl: Add a 565 pbuffer-only EGL config under X11.")
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit fc21394b)