msm: mdss: Account for null ctl passed to line count

Line count could be passed a null ctl structure,
which would cause a kernel panic. To prevent this,
line count will just return 0 if a null parameter
is passed.

Change-Id: I2b52533e6b67b842542de5b81be5b43fccc364dc
CRs-Fixed: 551701
Signed-off-by: Casey Piper <cpiper@codeaurora.org>