msm: mdss: access copy of userspace buf only

Copy buffer data from userspace and access only the
pointer to buffer in kernel memory. Copy back this data
back to user space memory once the operation is completed.
This prevents accessing user space virtual address from
driver.

Change-Id: I319bf08055cdf914803c4a35c5a607f92017d9a0
CRs-Fixed: 520129
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>