drm/fbdev-helper: don't force restores

Instead check for master status, in case we've raced.

This is the last exception to the general rule that we restore fbcon
only when there's no master active. Compositors are supposed to drop
their master status before they switch to a different console back to
text mode (or just switch to text mode directly, without a vt switch).

This is known to break some subtests of kms_fbcon_fbt in igt, but they're
just wrong - it does a graphics/text mode switch for the vt without
updating the master status.

Also add a comment to the drm_client->restore hook that this is expected
going forward from all clients (there's currently just one).

v2: Also drop the force in pan_display

v3: Restore the _force to pan_display, this actually means _locked in that
path. Spotted by Noralf.

Cc: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204150146.2006481-3-daniel.vetter@ffwll.ch