msm: mdss: Avoid null pointer dereferences

Null check on mfd param in overlay api needs to be performed
to avoid kernel panic.

CRs-fixed: 498973
Change-Id: If37284ab83bc678714fd8eed20f6adcb689846fa
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>

Conflicts:
	drivers/video/msm/mdss/mdss_mdp_overlay.c