drm/i915/tgl/dsi: Gate the ddi clocks after pll mapping

For TGL, there is no need to keep DDI clock on till IO enabling
for mipi dsi.

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730073648.5157-5-vandita.kulkarni@intel.com