msm: fbdev: add safety check for msm fb data object

Add NULL pointer safety check on msm fb data object to
prevent NULL access in rare display user-space usecase
of launching multiple window managers simultaneously.

Change-Id: Ib8baf78b2cbaceb19569693ad9faf53f6b2d1c9f
Signed-off-by: Chirag Khurana <ckhurana@codeaurora.org>