msm: fb: Fix security bugs in msmfb_notify_update & msm_fb_ioctl

msmfb_notify_update: Correct signature of msmfb_notify_update from
the ioctl declaration.

msm_fb_ioctl: Add appropriate null check for mfd data structure
before accessing it's members. This prevents null pointer accesses
in the driver.

CRs-Fixed: 526286
Change-Id: I731018057214fb0fa58c7617934100d2425617f8
Signed-off-by: Neti, Ravi Kumar <ravineti@codeaurora.org>