mesa: skip level checking for FramebufferTexture*D if texture is zero

mesa: skip level checking for FramebufferTexture*D if texture is zero

From the OpenGL 4.5 core spec:
  "An INVALID_VALUE error is generated if texture is not zero and level is
  not a supported texture level for textarget, as described above."

Other FramebufferTexture functions already do the right thing.

This fixes the main menu in F1 2015.

Cc: 11.1 11.2 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>