msm: mdss: validate the buffer size before allocating memory

There is no validation of the "count" parameter, which is controlled
by the user and used as a size of allocated memory. If the user
provides a value of "0" for "count", then kmalloc would not return
NULL, but also there will be a memory block of "zero" size. This can
lead to buffer overflows. Also trying to access invalid memory will
cause kernel crashes. Ensure to check that the number of bytes to be
written is non-zero. If zero, return invalid input.

Change-Id: I9613043881a91fd5a5f99337119c4a3d41493b54
Signed-off-by: Ashish Garg <ashigarg@codeaurora.org>