mesa: Support more glGet enums for ES3

mesa: Support more glGet enums for ES3

For glGetIntegerv, add support for the following in an OpenGL ES 3.0
context:
    GL_MAJOR_VERSION
    GL_MINOR_VERSION
    GL_NUM_EXTENSIONS

See Table 6.29 of the OpenGL ES 3.0 spec.

Fixes error GL_INVALID_ENUM in piglit egl-create-context-verify-gl-flavor,
testcase for OpenGL ES 3.0.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>