drm/i915: Use the passed in encoder

Just use the passed in encoder instead of digging it out via
the legacy drm_connector->encoder pointer (which we'll want to
stop using).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204180549.1267-5-ville.syrjala@linux.intel.com
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>