Input: evdev - only signal polls on full packets

This patch modifies evdev so that it only becomes readable when
the buffer contains an EV_SYN/SYN_REPORT event.

On SMP systems, it is possible for an evdev client blocked on poll()
to wake up and read events from the evdev ring buffer at the same
rate as they are enqueued.  This can result in high CPU usage,
particularly for MT devices, because the client ends up reading
events one at a time instead of reading complete packets.

We eliminate this problem by making the device readable only when
the buffer contains at least one complete packet.  This causes
clients to block until the entire packet is available.

Signed-off-by: Jeff Brown <jeffbrown@android.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>