Input: synaptics - allocate 3 slots to keep stability in image sensors

When slowly dropping 1, 2 and then 3 fingers on an image sensor touchpad,
we can see that the first finger gets reassigned a new slot while it did
not move. This is due to the kernel tracking algorithm which can not assign
correctly the 3 touches, being out of slots.

Declaring that we support 3 slots allows to actually forward:
slot 0 -> down, slot 1 -> up, slot 2 -> down

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Henrik Rydberg <rydberg@bitmath.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/synaptics.c

@@ -1189,7 +1189,7 @@ static void set_input_params(struct psmouse *psmouse,
 					ABS_MT_POSITION_Y);
 		/* Image sensors can report per-contact pressure */
 		input_set_abs_params(dev, ABS_MT_PRESSURE, 0, 255, 0, 0);
-		input_mt_init_slots(dev, 2, INPUT_MT_POINTER | INPUT_MT_TRACK);
+		input_mt_init_slots(dev, 3, INPUT_MT_POINTER | INPUT_MT_TRACK);
 
 		/* Image sensors can signal 4 and 5 finger clicks */
 		__set_bit(BTN_TOOL_QUADTAP, dev->keybit);