modesetting: re-set the crtc's mode when link-status goes BAD

Despite all the careful planning of the kernel, a link may become
insufficient to handle the currently-set mode. At this point, the
kernel should mark this particular configuration as being broken
and potentially prune the mode before setting the offending connector's
link-status to BAD and send the userspace a hotplug event. This may
happen right after a modeset or later on.

Upon receiving a hot-plug event, we iterate through the connectors to
re-apply the currently-set mode on all the connectors that have a
link-status property set to BAD. The kernel may be able to get the
link to work by dropping to using a lower link bpp (with the same
display bpp). However, the modeset may fail if the kernel has pruned
the mode, so to make users aware of this problem a warning is outputed
in the logs to warn about having a potentially-black display.

This patch does not modify the current behaviour of always propagating
the events to the randr clients. This allows desktop environments to
re-probe the connectors and select a new resolution based on the new
(currated) mode list if a mode disapeared. This behaviour is expected in
order to pass the Display Port compliance tests.

Signed-off-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit bcee1b76)