Add semi-mt support.

Kernel drivers for 'semi-mt' devices, such as the Synaptics profile
sensor, do not provide a pressure value with their semi-mt slots.
Instead, they report a single pressure value via ABS_PRESSURE. So, if
the device's INPUT_PROP_SEMI_MT bit is set we use the ABS_PRESSURE value
to set the height/width of all plotted circles.

BUG=chromium-os:24277
TEST=mtplot
And see if the touch events could be drawn on Cr-48.

Change-Id: I4c3d564fc7fd902b7cdae0ac5850730f07ffde86