drm/msm/dsi-staging: rename ctrl index to cell_index

Currently display control structure has a member "index" that
represents cell_index. Patch renames that to "cell_index" to
reflect correct meaning of that member and do not mix it with
controller index in the clock manager array.

Change-Id: Ia9047a500ac0ce9987cdc06e9564dad9efe45479
Signed-off-by: Alexander Beykun <abeykun@codeaurora.org>