drm/msm/sde: return last misr status during idle pc

Certain SDE clients like ad4 may want to collect the last
misr status during idle power collapse state. MISR enable
should not leave the leave the mdp clock in enabled state
because it may conflict with ad4 + retention mode. This patch
stores the last misr state on layer mixer and returns to
client when clocks are off. It also re-enable this feature
during power restore state.

Change-Id: I9bc66bed25640230169c11cd9c398dc0a1eba9c8
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>