msm_ts: ignore noisy low pressure touches

msm touchscreen gives noisy x,y positions on light presses. ignore light
presses using an arbitrary cut-off pressure of 5/255.

by @plaguedbypenguins

Change-Id: Ibf5473e921615f299317c745a9ebcb589ad283c9
Signed-off-by: Michael Bestas <mikeioannina@gmail.com>