msm: mdss: Check null-pointer dereferencing and buffer overflows

Add checks before possible null-pointer dereferencing. Add checks to
prevent buffer out-of-bounds access.

Change-Id: I17419c82654c4ad2d18c7f88e34447cf333450bb
Signed-off-by: Mathew Karimpanal <kjmathew@codeaurora.org>
Signed-off-by: Beeresh Gopal <gbeeresh@codeaurora.org>