drm: Simplify fb refcounting rules around ->update_plane

The introduction of primary planes has apparently caused a bit of fb
refcounting fun for people. That makes it a good time to clean up the
arcane rules and slight differences between ->update_plane and
->set_config. The new rules are:

- The core holds a reference for both the new and the old fb (if
  they're non-NULL of course) while calling into the driver through
  either ->update_plane or ->set_config.

- Drivers may not clobber plane->fb if their callback fails. If they
  do that, they need to store a pointer to the old fb in it again.
  When calling into the driver plane->fb still points at the current
  (old) framebuffer.

- The core will update the plane->fb pointer on success. Drivers can
  do that themselves too, but aren't required to any more for the
  primary plane.

- The core will update fb refcounts for the plane->fb pointer,
  presuming the drivers hold up their end of the bargain.

v2: Remove now unused tmpfb (Thierry)

v3: Drop broken changes from drm_mode_setplane (Ville). Also polish
the commit message a bit.

v4: Also fix up the handling of ->disable_plane in
drm_plane_force_disable. The issue was that we didn't save plane->fb
over the ->disable_plane call. Just paranoia, nothing relies on this.

v5: Keep still useful comments about directly calling ->set_config,
which I should have done for v4 already. Requested by Matt.

Cc: Thierry Reding <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Matt Roper <[email protected]>
Reviewed-by: Matt Roper <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>

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