mesa: Fix GL error generation in _mesa_GetFramebufferAttachmentParameteriv()

mesa: Fix GL error generation in _mesa_GetFramebufferAttachmentParameteriv()

This allows query on default framebuffer in
glGetFramebufferAttachmentParameteriv() for gles3. Fixes unexpected GL
errors in gles3 conformance test case:
framebuffer_blit_functionality_multisampled_to_singlesampled_blit

V2: Use _mesa_is_gles3() check to restrict allowed attachment types to
specific APIs.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>