dri: Define enum __DRI_API_GLES3

dri: Define enum __DRI_API_GLES3

This enum corresponds to EGL_OPENGL_ES3_BIT_KHR.
Neither the GLX nor EGL layer use the enum yet.

I don't like the GLES bits. I'd prefer that all GLES APIs be exposed
through a single API bit, as is done in GLX_EXT_create_context_es_profile.
But, we need this GLES3 enum in order to do the plumbing necessary to
correctly support EGL_OPENGL_ES3_BIT_KHR as required by the
EGL_KHR_create_context spec.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>