drm/fb-helper: Fix vt restore

In the past we had a pile of hacks to orchestrate access between fbdev
emulation and native kms clients. We've tried to streamline this, by
always preferring the kms side above fbdev calls when a drm master
exists, because drm master controls access to the display resources.

Unfortunately this breaks existing userspace, specifically Xorg. When
exiting Xorg first restores the console to text mode using the KDSET
ioctl on the vt. This does nothing, because a drm master is still
around. Then it drops the drm master status, which again does nothing,
because logind is keeping additional drm fd open to be able to
orchestrate vt switches. In the past this is the point where fbdev was
restored, as part of the ->lastclose hook on the drm side.

Now to fix this regression we don't want to go back to letting fbdev
restore things whenever it feels like, or to the pile of hacks we've
had before. Instead try and go with a minimal exception to make the
KDSET case work again, and nothing else.

This means that if userspace does a KDSET call when switching between
graphical compositors, there will be some flickering with fbcon
showing up for a bit. But a) that's not a regression and b) userspace
can fix it by improving the vt switching dance - logind should have
all the information it needs.

While pondering all this I'm also wondering wheter we should have a
SWITCH_MASTER ioctl to allow race-free master status handover. But
that's for another day.

v2: Somehow forgot to cc all the fbdev people.

v3: Fix typo Alex spotted.

Reviewed-by: Alex Deucher <[email protected]>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=208179
Cc: [email protected]
Reported-and-Tested-by: [email protected]
Cc: Michel Dänzer <[email protected]>
Fixes: 64914da24ea9 ("drm/fbdev-helper: don't force restores")
Cc: Noralf Trønnes <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # v5.7+
Cc: Bartlomiej Zolnierkiewicz <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Qiujun Huang <[email protected]>
Cc: Peter Rosin <[email protected]>
Cc: [email protected]
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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