Input: alps - use struct input_mt_pos to track coordinates

This is a preparation patch for switching the DIY mt handling to using
input_mt_assign_slots && input_mt_sync_frame.

struct alps_fields is quite large, so while making changes to almost all uses
of it lets put it in our priv data instead of on the stack.

Having it in our priv data also allows using it directly for storing values
which need to be cached, rather then having separate x, y, z, fingers, etc.
copies in our priv data.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>