msm: mdss: use uaccess routines to access user space buffers

Use routines defined in uaccess.h (like put_user, get_user)
while accessing user space buffers. Otherwise this leads to
userspace access violation when PAN is enabled.

Change-Id: Iea16e347ee71f0845711a21b6b9b01e16a6ab619
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>