xv: Move common code for adaptor cleanup to xvmain.c

Since any DDX XV screen cleanup would need this same code for freeing
the tree of pointers for xv adaptors, move it to the dix.

v2: Unconditionalize the pPorts freeing, to match the block above it.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Keith Packard <keithp@keithp.com> (v1)