egl: Make the 565 pbuffer-only config single buffered.

In commit dacb11a5, Eric found the first
matching 565 pbuffer config, and stopped.  Our double-buffered configs
come first in the list, so we added that, making a pbuffer-only config
that claimed to be double buffered.  This doesn't make sense, since
pixmaps/pbuffers are fundamentally not double buffered.

When using that config, every call to eglCreatePbufferSurface would fail
with EGL_BAD_MATCH.  The call chain looks like this:

   - eglCreatePbufferSurface
   - dri3_create_pbuffer_surface
   - dri3_create_surface
   - dri2_get_dri_config

which eventually does:

   const bool double_buffer = surface_type == EGL_WINDOW_BIT;

and then fails to find a matching config, because it ends up looking
for a single-buffered config - and there aren't any.

To fix this, make the 565 pbuffer config single-buffered.  This fixes
at least 51 dEQP-EGL.* tests.

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 6ad31c4f)