drm/msm/sde: disable power only after reading the line count

Disable power after the VSYNC event worker has read the current
line count register and calculated the new wakeup time.

Change-Id: I9e5583b1ac189f683d90ee2e906e0cd36ecbab68
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>