Input: Send events one packet at a time

Input: Send events one packet at a time

On heavy event loads, such as a multitouch driver, the irqsoff latency
can be as high as 250 us.  By accumulating a frame worth of data
before passing it on, the latency can be dramatically reduced.  As a
side effect, the special EV_SYN handling can be removed, since the
frame is now atomic.

This patch adds the events() handler callback and uses it if it
exists. The latency is improved by 50 us even without the callback.

Cc: Daniel Kurtz <[email protected]>
Tested-by: Benjamin Tissoires <[email protected]>
Tested-by: Ping Cheng <[email protected]>
Tested-by: Sedat Dilek <[email protected]>
Acked-by: Dmitry Torokhov <[email protected]>
Signed-off-by: Henrik Rydberg <[email protected]>