llvmpipe: initialize default fb correctly in setup

llvmpipe: initialize default fb correctly in setup

If lp_setup_bind_framebuffer() is never called, then setup fb x1/y1 was not
correctly initialized. This can happen if there's never a fb set - both
cso and llvmpipe would consider setting this with no cbufs and no zsbuf a
redundant change and therefore it would never get set.
We rely on this setup fb rect being initialized correctly for the tri intersect
tests, throwing away tris which don't intersect. Not initializing it meant
we'd then say it intersected, and we'd try to bin that despite that we have
no actual tiles to bin it to, leading to assertion failures (pretty harmless
since tile 0/0 always exists nevertheless as tiles are statically allocated,
albeit that should change at some point).
(Note probably not an issue with gl state tracker)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>