disp: msm: sde: add check before buffer copy

This change adds a check for the length of the buffer
before copying it to avoid a buffer overflow.

Change-Id: I9af9d422e0b3cf02c8d6662af3310337a9861a7a
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>