msm: cvp: avoid OOB write while accessing memory

Exclude 4 bytes which holds the size of the buffer while calculating
the actual buffer size to avoid OOB write.

Change-Id: I658eab26c8e399bcda94e4e570b9a08dbb388cea
Signed-off-by: suchawla <suchawla@codeaurora.org>
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>