drm/msm/sde: prepare only affected encoder for commit

Currently all connected encoders are prepared on each
commit even though it is for crtc not specified in the
commit state. This causes these extra encoders to be
prepared incorrectly. Correct prepare commit to loop
thru only those crtcs specified in the commit state,
and only prepare those encoders connected.

Change-Id: Id14380b33882787a6b5f8affcd8ffcbc3e86bbec
Signed-off-by: Alan Kwong <akwong@codeaurora.org>