msm: mdss: always set mdp_busy when partial update is disabled

mdp_busy flag is used to serialze kickoff operations by ensuring
that any pending kickoffs are first completed before issuing any
new kickoffs. This avoids any potential mpd_busy_wait timeouts.
Therefore, for normal operations (partial update disabled), mdp_busy
flag should always be set. However, when partial update is enabled,
width and height of panel's ROI need to be checked before setting
mdp_busy flag. Otherwise mdp_busy_wait will timeout since no update
will be sent to the panel. In addition, perform roi merge from both
panels if roi-merge is enabled to decide whether column and page address
has changed. If it was changed, then new column and page address need
to be sent to the panel.

CRs-Fixed: 692382
Change-Id: Ib0070b84a626afffd4c51a65f59bc97aae1d8994
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>