HID: multitouch: handle palm for touchscreens

Usually, there is no palm rejection for touchscreens. You don't rest
your palm on the touchscreen while interacting with it.
However, some wacom devices do so because you can rest your palm while
interacting with the stylus.

Unfortunately, the spec for touchscreens[1] is less precise than the one
for touchpads[2]. This leads to a situation where it's 'legitimate'
for a touchscreen to provide both tipswitch off and confidence off in the
same report.

Work around that by keeping the slot active for one frame where we report
MT_TOOL_PALM, and then synthesizing the release event in a separate frame.
frame

Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
[rebased and new commit message]
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>