mdss: mdp: Update histogram start failure reporting

Userspace clients will enable the histogram and disable it as per the
algorithm requirements. If the process under which the client is
running crashes and restarts states of histogram between client and
driver is not in sync. In case the driver is not in sync with client it
should return EBUSY error to client. Client will stop and start the
histogram to maintain the sync.

Change-Id: I7b3c8d7890d8ca24ecf065e48a9ea5884cc886de
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>