llvmpipe: Recompute the fs shader key when framebuffer varies.

llvmpipe: Recompute the fs shader key when framebuffer varies.

The fs shader now depends on the color buffer formats. The shader key was
extended to accommodate this, but llvmpipe_update_derived needs to be
updated to check the framebuffer dirty flag.

This fixes bug 57674.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>