msm: camera: isp: Prevent out of bounds read

When command buffers are passed from userspace in the form
of generic blobs, the size must be validated to prevent out
of bounds read.

Change-Id: Ifd7ec00c8923650d9c86322c102ed0c724748445
Signed-off-by: Venkat Chinta <vchinta@codeaurora.org>