drm/i915: Polish intel_get_lvds_encoder()

Pass dev_priv to intel_get_lvds_encoder() and polish the
implementation a bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190318202653.15217-2-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>