Input: evdev - convert to dynamic event buffer

Allocate the event buffer dynamically, and prepare to compute the
buffer size in a separate function. This patch defines the size
computation to be identical to the current code, and does not contain
any logical changes.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>