msm: mdss: decouple histogram stop from control mixers

When histogram is started for a logical display we store the display
number as part of histogram info structure. When histogram stop is
called on a logical display control mixers might be turned off in case
of suspend usecase which will fail histogram stop ioctl. Since driver is
caching the display number, histogram for the dspp's which are attached
to logical display can be turned off without need for checking control
mixers.

Change-Id: I4f06f5aad562ff3a3df5449166b7d174d3e800eb
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>
Signed-off-by: Ping Li <pingli@codeaurora.org>