The intention of using video=<connector>:<mode> is primarily to select
the user's preferred resolution at startup. Currently we always create a
new mode irrespective of whether the monitor has a native mode at the
desired resolution. This has the issue that we may then select the fake
mode rather the native mode during fb_helper->inital_config() and so
if the fake mode is invalid we then end up with a loss of signal. Oops.
This invalid fake mode would also be exported to userspace, who
potentially may make the same mistake.

To avoid this issue, we filter out the added command line mode if we
detect the desired resolution (and clock if specified) amongst the
probed modes. This fixes the immediate problem of adding a duplicate
mode, but perhaps more generically we should avoid adding a GTF mode if
the monitor has an EDID that is not GTF-compatible, or similarly for
CVT.

Was meant to fix a regression from

commit eaf99c74


Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Aug 6 10:08:32 2014 +0200

    drm: Perform cmdline mode parsing during connector initialisation

but Radek explained that the original bug is no longer reproducible on
latest kernels.

v2: Explicitly delete our earlier cmdline mode
v3: Mode pruning should now be sufficient to delete stale cmdline modes
v4: Compute the vrefresh for the probed mode

Reported-by: Radek Dostál <rd@radekdostal.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Radek Dostál <rd@radekdostal.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: Julia Lemire <jlemire@matrox.com>
Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Drop cc: stable since no longer a pressing bugfix, just
nice-to-have.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464774651-20376-1-git-send-email-chris@chris-wilson.co.uk