drm/msm/sde: bypass crtc atomic begin/flush if disabled

Current crtc exeuctes atomic begin and flush even if crtc
is not enabled.  This causes unnecessary updates to crtc.
Hence, modify crtc to bypass atomic begin and flush if not
enabled.

Change-Id: I1de9c0965e552126ed7be59f7ee5400e3605a17f
Signed-off-by: Alan Kwong <akwong@codeaurora.org>