The following commit regressed lookup of some new OpenGL symbols:

    commit cb334db3
    Author: Daniel Kurtz <djkurtz@chromium.org>
    Date:   Thu Oct 16 21:36:01 2014 +0800
    Subject: dispatch: Use dlsym to lookup core symbols for EGL (v2)

It regressed these Piglit tests on i965:
    spec@arb_pipeline_statistics_query@arb_pipeline_statistics_query-vert_adj
    spec@arb_pipeline_statistics_query@arb_pipeline_statistics_query-geom

Fix the tests by consistently using egl/glXGetProcAddress to lookup core
OpenGL symbols instead of dlsym(), and add a lengthy comment explaining
why. (For core OpenGL ES symbols, continue using dlsym() as in
cb334db3

).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Mark Janes <mark.a.janes@intel.com>