msm: mdss: fix null pointer dereferencing

Current driver checks if the address of start pointer is zero. But
in cases when start pointer is NULL this check will result in null
pointer dereferencing. This change adds an additioanl check on the
start pointer to avoid null pointer dereferencing.
CRs-fixed: 2035100
Change-Id: Ic86c9e73cd676cbf8680f5b7266ee3829f4ccbd2
Signed-off-by: Harsh Sahu <hsahu@codeaurora.org>
Signed-off-by: Mishra Mahima <mahima@codeaurora.org>