msm: mdss: Return success to hist ON ioctl if running in mdp3

Request to start histogram when it is already running is
returned as error. It should not be an error and should
return success. Also a log should be printed to mention
that histogram was already running for user information.

Change-Id: I74df34574431f4516e0959fa33c3a6b548c88160
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>