glx: Query proper extension string for ARB_create_context etc.

Previously we were using glXQueryExtensionsString() to see whether the
implementation supported ARB_create_context,
ARB_create_context_profile, and EXT_create_context_es_profile.  This
is the wrong query--those extensions show up in the *client* extension
string, so we need to use glXGetClientString().

This allows us to create core contexts using GLX.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>