ventana: use pendown handler from tsc2007

As our tsc2007 has an interrupt back to the CPU it makes more sense to
provide a pendown handler that uses the state of this pin to determine
if the pen is up or down instead of trying to caculate it using x_plat_ohms
and dealing with preasure sensitivity.

This resolves sensitivity issues with the tsc2007 on the GW16109 display board.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Revert "tsc2007: add option to invert pressure"

This reverts commit b1baf0c03196dcbab23eea5f56e1f5345086e0f1.