Input: synaptics - refactor agm packet parsing

When a Synaptics touchpad is in "AGM" mode, and multiple fingers are
detected, the touchpad sends alternating "Advanced Gesture Mode" (AGM) and
"Simple Gesture Mode" (SGM) packets.
  The AGM packets have w=2, and contain reduced resolution finger data.
  The SGM packets have w={0,1} and contain full resolution finger data.

Refactor the parsing of agm packets to its own function, and rename the
synaptics_data.mt field to .agm to indicate that it contains the contents of
the last agm packet.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Chase Douglas <chase.douglas@canonical.com>
Acked-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>