msm: vidc: Fix possible NULL pointer dereference

Added NULL pointer check for platform_data structure to avoid NULL
pointer dereference error. platform_data is initialized with return 
value (driver_data) of vidc_get_drv_data() fucntion where driver_data is 
initialized with NULL pointer at the beginning.

Change-Id: I24743cc6ed7ffcfa40feca2307c9b1728b5bc152
Signed-off-by: Mahesh Voorugonda <mvoorugo@codeaurora.org>