Input: atmel_mxt_ts - simplify event reporting

Instead of carrying around per-finger state in the driver instance, just
report each finger as it arrives to the input layer, and let the input
layer (evdev) hold the event state (which it does anyway).

Note: this driver does not really do MT-B properly. Each input report
(a group of input events followed by a SYN_REPORT) only contains data for
a single contact.  When multiple fingers are present on a device, each is
properly reported in its own MT_SLOT.  However, there is only ever one
MT_SLOT per SYN_REPORT.  This is fixed in a subsequent patch.

This patch was tested with an mXT224E.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>