table: move custom rotation handling into a helper function

table: move custom rotation handling into a helper function

This is a bit hard to follow:
- tilt is handled first and if either tilt axis is set we fetch *both* tilt axes
  into tablet->axes.tilt
- rotation is handled second but it only triggers if either tilt axis is
  flagged. as we now guarantee to have both axes in tablet->axes.tilt, we
  can continue with the rotation conversion without needing some other state

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Ping Cheng <pingc@wacom.com>