msm: mdss: fix possible null dereference of fb index

Rotator data paths don't have a fb device assigned, assuming that it
will contains a reference to fd data is incorrect. Add check to avoid
NULL pointer dereference.

Change-Id: Ib91e96ddc56590f2f681673b162921191e3d86da
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>