drm/i915: dont trigger hotplug events on unchanged ELD

The ELD may or may not change when switching the video mode.
If unchanged, don't trigger hot plug events to HDMI audio driver.

This avoids disturbing the user with repeated printks.

Reported-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>