mesa/glsl/glapi: enable GL_EXT_draw_buffers extension

mesa/glsl/glapi: enable GL_EXT_draw_buffers extension

Patch enables ES2 extension that utilizes existing ES3 functionality.

Changes make all the subtests to run and pass in WebGL conformance
test 'webgl-draw-buffers' when running Chrome on OpenGL ES, also
Piglit test 'draw_buffers_gles2' passes.

v2: remove unused boolean (Ilia Mirkin)
v3: proper error checking for invalid values (Chad Versace)
v4: run error check explicitly for ES2 and ES3 (Kenneth Graunke)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>