gl-renderer: fix configless_context check

EGL_MESA_configless_context is a display extension. The query for client
extensions was overwriting the pointer, so it was being searched from
the client extensions instead.

Fix any confusion here by moving all client extension checks into
another function. Drop a useless cast.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>