msm: mdss: remove unnecessary blank event notification

FB blank notification is not needed because the event is already
being generated in core fb driver. This change avoids sending the
same notification twice.

Change-Id: I8c05424e6675d35958cfeedfb4428812728a8399
CRs-Fixed: 482340
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>