input: touchscreen: bu21150: enable IRQ after system is up

Currently IRQ is enabled after receiving EARLY BLANK event from
display. At this time, rest of the system is not ready and imposes
problems. Move IRQ enablement from FB callback into ioctl callback
to make sure system is up when IRQ is enabled.

Change-Id: I508b04e2e93e970c6a0a04d4590a98ac93c8b6f3
Signed-off-by: Chun Zhang <chunz@codeaurora.org>