dix: check the xi2mask, not the grab type for touch listeners

dix: check the xi2mask, not the grab type for touch listeners

grab->type is only non-zero for passive grabs. We're checking an active grab
here, so we need to check if the touch mask is set on the grab.

Test case: grab the device, then start two simultaneous touches. The
grabbing client won't see the second touchpoints because grab->type is 0
and the second touch is not an emulating pointer.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>