msm: mdss: fb: null check mfd data

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

Change-Id: Id301d4a90c9a52049944c7e3b31aa1a34ad292d7
CRs-Fixed: 526286
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>