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.

(Ported from xserver commit bcee1b76aa0db8525b491485e90b8740763d7de6)

[ Michel: Bump libdrm dependency to >= 2.4.78 for
  DRM_MODE_LINK_STATUS_BAD ]
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

configure.ac

@@ -70,7 +70,7 @@ XORG_DRIVER_CHECK_EXT(XV, videoproto)
 XORG_DRIVER_CHECK_EXT(DPMSExtension, xextproto)
 
 # Checks for libraries.
-PKG_CHECK_MODULES(LIBDRM, [libdrm >= 2.4.60])
+PKG_CHECK_MODULES(LIBDRM, [libdrm >= 2.4.78])
 PKG_CHECK_MODULES(LIBDRM_RADEON, [libdrm_radeon])
 
 # Obtain compiler/linker options for the driver dependencies