modesetting: keep going if a modeset fails on EnterVT

There was a time when setting a mode on a CRTC would not depend on the
associated connector's state. If a mode had been set successfully once,
it would mean it would work later on.

This changed with the introduction of new connectors type that now
require a link training sequence (DP, HDMI 2.0), and that means that
some events may have happened while the X server was not master that
would then prevent the mode from successfully be restored to its
previous state.

This patch relaxes the requirement that all modes should be restored on
EnterVT, or the entire X-Server would go down by allowing modesets to
fail (with some warnings). If a modeset fails, the CRTC will be
disabled, and a RandR event will be sent for the desktop environment to
fix the situation as well as possible.

Additional patches might be needed to make sure that the user would
never be left with all screens black in some scenarios.

v2 (Martin Peres):
 - whitespace fixes
 - remove the uevent handling (it is done in a previous patch)
 - improve the commit message
 - reduce the size of the patch by not changing lines needlessly
 - return FALSE if one modeset fails in ignore mode
 - add comments/todos to explain why we do things
 - disable the CRTCs that failed the modeset

Signed-off-by: Kishore Kadiyala <kishore.kadiyala@intel.com>
Signed-off-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Kishore Kadiyala <kishore.kadiyala@intel.com>
Closes: #1010
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