drm/format-helper: Pass format-conversion state to helpers

Pass an instance of struct drm_format_conv_state to DRM's format
conversion helpers. Update all callers.

Most drivers can use the format-conversion state from their shadow-
plane state. The shadow plane's destroy function releases the
allocated buffer. Drivers will later be able to allocate a buffer
of appropriate size in their plane's atomic_check code.

The gud driver uses a separate thread for committing updates. For
now, the update worker contains its own format-conversion state.

Images in the format-helper tests are small. The tests preallocate
a static page for the temporary buffer. Unloading the module releases
the memory.

v6:
	* update patch for ssd132x support
v5:
	* avoid using unusupported shadow-plane state in repaper (Noralf)
	* fix documentation (Noralf, kernel test robot)
v3:
	* store buffer in shadow-plane state (Javier, Maxime)
	* replace ARRAY_SIZE() with sizeof() (Jani)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Javier Martinez Canillas <javierm@redhat.com> # ssd130x
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231009141018.11291-4-tzimmermann@suse.de