FB helper code falls back to a 1024x768 mode if no outputs are connected
or don't report back any modes upon initialization. This can be annoying
because outputs that are added to FB helper later on can't be used with
FB helper if they don't support a matching mode.

The fallback is in place because VGA connectors can happen to report an
unknown connection status even when they are in fact connected.

Some drivers have custom solutions in place to defer FB helper setup
until at least one output is connected. But the logic behind these
solutions is always the same and there is nothing driver-specific about
it, so a better alterative is to fix the FB helper core and add support
for all drivers automatically.

This patch adds support for deferred FB helper setup. It checks all the
connectors for their connection status, and if all of them report to be
disconnected marks the FB helper as needing deferred setup. Whet setup
is deferred, the FB helper core will automatically retry setup after a
hotplug event, and it will keep trying until it succeeds.

v2: Rebase onto my entirely reworked fbdev helper locking. One big
difference is that this version again drops&reacquires the fbdev lock
(which is now fb_helper->lock, but before this patch series it was
mode_config->mutex), because register_framebuffer must be able to
recurse back into fbdev helper code for the initial screen setup.

v3: __drm_fb_helper_initial_config must hold fb_helper->lock upon
return, I've fumbled that in the deferred setup case (Liviu).

v4: I was blind, redo this all. __drm_fb_helper_initial_config
shouldn't need to reacquire fb_helper->lock, that just confuses
callers. I myself got confused by kernel_fb_helper_lock and somehow
thought it's the same as fb_helper->lock. Tsk.

Also simplify the logic a bit (we don't need two functions to probe
connectors), we can stick much closer to the existing code. And update
some comments I've spotted that are outdated.

v5: Don't pass -EAGAIN to drivers, it's just an internal error code
(Liviu).

v6: Add _and_unlock suffix to clarify locking (Maarten)

Cc: Liviu Dudau <liviu@dudau.co.uk>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com> (v1)
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@intel.com>
Reviewed-by: Liviu Dudau <liviu@dudau.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170706130023.28417-3-daniel.vetter@ffwll.chSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>