drm/i915: Push TRANS_DDI_FUNC_CTL into the encoder->enable() hook

Push the TRANS_DDI_FUNC_CTL into the encoder enable hook. The disable
is already there, and as a followup will enable us to pass the encoder
all the way down.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200417134720.16654-3-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jani Nikula <jani.nikula@intel.com>