drm/i915: only report hpd connector status change when it actually changed

This reduces dmesg noise when there's a glitch on the hpd line, or there
are more than one connectors on the same hpd line and only one of them
changes.

While at it, switch to use the friendly status names instead of numbers.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>