mesa: Accept GL_BACK in get_fb0_attachment with ARB_ES3_1_compatibility.

According to the ARB_ES3_1_compatibility specification,
glGetFramebufferAttachmentParameteriv is supposed to accept BACK,
and it behaves exactly like BACK_LEFT.

Fixes a GL error in GFXBench 5 Aztec Ruins.

Cc: "17.3 17.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
(cherry picked from commit 4f538c3f)