target-helpers: remove swrast_xlib.c helper

target-helpers: remove swrast_xlib.c helper

This was a good idea, but ended up tying the build systems in knots.
We can revisit this later, in particular if we can put in place dummy
implementations of cell_create_screen(), llvmpipe_create_screen()
which just return NULL if the driver isn't available.

In the meantime, just duplicate this smallish function in the two
places it was being called.