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: I8919f0b01dcbcad9300c28e070e9b9ca64368db2
Signed-off-by: Narendra Muppalla <NarendraM@codeaurora.org>