drm/i915: Implement the missing bits of assert_panel_unlocked()

Add the missing eDP port handling into assert_panel_unlocked(). We now
have intel_dp_port_enabled() which makes this trivial.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518152931.13104-6-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>