drm/msm/sde: update mode-2 entry/exit sequence

Disable mode-2 selection from RSC hardware after
sw has triggered it manually. This avoids double
shutdown issue if panel does not generate a vsync
after solver enabled. The change also updates F0
Qtimer trigger for mode_2 entry triggered during
mode_0 or mode_1.

Change-Id: Ic7a993acbdb7cdeb4ff9654846b430cdf0938d7a
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>