mdss: mdp: prevent null pointer dereference in mixer width calculation

Frame buffer releases all allocated control nodes by calling
blank when its reference count reaches to zero. In error
case, caller might call prepare ioctl and lead to null
pointer dereference to calculate the layer mixer width. A null
condition check prevents such dereferencing call.

CRs-fixed: 664517
Change-Id: I3b65b48a67ab2efb9654457ca7901da6ef0d57be
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>