drm/msm/dsi-staging: Add pre/post clk off/on callback path

Add pre clk off and post clk off sequence while disabling dsi
link clk and pre clk on and post clk on sequence while enabling
dsi link clk. This fixes the warning logs while switching the
dsi clk via sysfs interface.

Change-Id: Icb751dc369bed1119506275e33f30b3ed3a713b3
Signed-off-by: Ritesh Kumar <riteshk@codeaurora.org>