drm/msm/sde: move lp handling to kickoff callback

Move updates for the SDE connector's LP property from the
atomic property set to the commit kickoff callback. This
prevents calls to the underlying display for atomic
TEST_ONLY requests.

Change-Id: I4edf98b78ee0800847fffb4395c00fa3df91a061
Signed-off-by: Clarence Ip <cip@codeaurora.org>