drm/msm/sde: clean up crtc ctl tracking

Remove duplicate ctl flush mask cache in the CRTC since it no
longer needs to maintain a separate cache.

Clean up assignment of ctl to mixer assignments to reflect that
two mixers may be controlled by the same ctl.

Change-Id: Id6db2d69e60a5728e026307685e60a0c52220927
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>