drm/msm/sde: count null character for blob length

Return value of snprintf doesn't account for the terminating
null character it appends to the end of every string.
So this change adds one extra character length to the blob size
to avoid over reading from the client end.

Change-Id: I4aa74b4c49171d642829b331a88f5960a7d25793
CRs-Fixed: 2044242
Signed-off-by: Narendra Muppalla <NarendraM@codeaurora.org>
Signed-off-by: Yujun Zhang <yujunzhang@codeaurora.org>