dri3: Fix MakeCurrent without a default framebuffer
In OpenGL 3.0 and later it is legal to make a context current without
a default framebuffer.
This has been broken since DRI3 support was introduced.
Cc: "13.0 12.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by:
Marek Olšák <marek.olsak@amd.com>