msm: mdss: Return success to hist start if hist is running

When histogram is already running a subsequent call to start
the histogram would return with an error. Now instead of
failing the request we return success if histogram is found
to be running already.

Change-Id: I728d8388be625ff5d8069166ad4517095a71727b
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>