drm/i915/ddi: Push pipe clock enabling to encoders

On ICL the pipe clock needs to be enabled before setting the HDMI
infoframe, but these steps are in the reverse order atm. Move the pipe
clock enabling to the encoders, so reordering of the two steps can be
done in a clean way.

No functional change.

v2:
- Rebased on drm-tip.

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180613172746.18525-1-imre.deak@intel.com