drm/i915/dsi: Enable ICL DSI transcoder as part of encoder->enable

drm/i915/dsi: Enable ICL DSI transcoder as part of encoder->enable

Enable the dsi transcoder, panel and backlight as part of
encoder->enable and not encoder->pre_enable. We need to have pipe src
size, among other things, set before enabling the transcoder, to avoid
FIFO underruns and possibly other issues.

v2 by Jani:
- Rebase on the crtc enable sequence update

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128162850.8660-2-jani.nikula@intel.com