evdev: send old valuator inside event frame when the other is missing

evdev: send old valuator inside event frame when the other is missing

when a motion is being performed on ts device, only one axis can be sent
through the evdev bytestream whereas the other could be omitted. For instance:

   -------------- SYN_REPORT ------------
   type 3 (EV_ABS), code 53 (ABS_MT_POSITION_X), value 22208
   type 3 (EV_ABS), code 58 (ABS_MT_PRESSURE), value 631
   type 3 (EV_ABS), code 0 (ABS_X), value 22208
   -------------- SYN_REPORT ------------

on such case we'd have to send the compositor the old value of Y. Commit
f547bd36 introduced this bug cause it was sending zeroed coordinate and not
the old one.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>