msm: mdss: add validation for dest scaler count

User space can send the commit message with the dest scaler
structure populated and dest scaler count as 0, this would
cause null pointer access, this change adds validation for both
the fields.

Change-Id: I7a4ad3188f7a19427c096a596a502debdc2aac55
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>