drm/msm/sde: update frame trigger mode debugfs property

Update frame trigger mode debugfs property as read
only property. DRM client can change this property
through connector property on each frame kickoff.
Debugfs write option may cause race condition with
DRM client setting.

Change-Id: I4375455e8ea1da73a71a6ce663fcc93ae1a0612e
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>