drm/msm/dp: changes to prevent null pointer dereference

Change the buffer size to fix a possible null pointer access

Change-Id: I457d66e9ebe3902a68367a5f0556cf65b617db93
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>