st/mesa: Fix recurring surfaceless contexts

st/mesa: Fix recurring surfaceless contexts

A current incomplete framebuffer was incorrectly used as a
st_framebuffer. When accessing st_framebuffer childs bad things happen:
e.g. st_framebuffer::iface was used to check whether its an incomplete
fb, instead we need to compare st_framebuffer::Base against
mesa_get_incomplete_framebuffer.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44919
Note: This is a candidate for the 8.0 branch.

Reviewed-by: Brian Paul <brianp@vmware.com>