mesa: Don't advertise GLES extensions in GL contexts

mesa: Don't advertise GLES extensions in GL contexts

glGetStringi(GL_EXTENSIONS) failed to respect the context's API, and so
returned all internally enabled GLES extensions from a GL context.
Likewise, glGetIntegerv(GL_NUM_EXTENSIONS) also failed to repsect the
context's API.

Note: This is a candidate for the 8.0 and 9.0 branches.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>