mdss: display: add dsi dynamic clock control

This patch adds dsi dynamic clock control support which is
based on reference count. Both dsi clock and mdp clock are
controlled together during screen update except dsi dcs
command which needs to control dsi clock independent of screen
update.

Change-Id: Ibece46ce374c80f4d6d55230efbee8a72b6e0c9e
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>