msm: mdss: Calculate histogram frame size using correct control paths

For a given logical display, use the correct control path to calculate
the frame size.  Previously, we were using a control path assuming the
first mixer is used for primary display.

Change-Id: Ic8b0c1ab9fb802825a0abdb334efac6ea376ad65
Signed-off-by: Benet Clark <benetc@codeaurora.org>