msm: vidc: add check to avoid out-of-buffer write

Possibility of dereferencing userspace ptr in kernel for
invalid cmd. So added check to return error if unsupported
cmd is given as input to ioctl.

Change-Id: I3466fbd06e5b600f748824b9e16bcfdb4438bdef
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>