msm: mdss: ensure rotator clocks on for entire rotator session

Rotator clock and BW votes are updated on each rotator request
during same session. This is not required because rotator source
and destination does not change during session. Ensure that
rotator clocks are on throughout session such that extra overhead
of rpm calls are avoided.

Change-Id: I7a74aa2b8d8556b1d8ee4c52f871b9ebb149764a
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>