disp: msm: check if kms is instantiated

This defensive check is now required since the allocation
of driver private data has moved from bind to probe phase.
There is a chance that device shutdown can be called before
bind phase and this can cause invalid memory access.

Change-Id: I2d471a4aff93ae03291e19f6f902f1e5c28dc50f
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>