drm/repaper: Drop explicit drm_mode_config_cleanup call

drm/repaper: Drop explicit drm_mode_config_cleanup call

Allows us to drop the drm_driver.release callback.

This is made possible by a preceeding patch which added a drmm_
cleanup action to drm_mode_config_init(), hence all we need to do to
ensure that drm_mode_config_cleanup() is run on final drm_device
cleanup is check the new error code for _init().

v2: Explain why this cleanup is possible (Laurent).

v3: Use drmm_mode_config_init() for more clarity (Sam, Thomas)
I also noticed that I've failed to add the error checking,
__must_check caught that.

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-47-daniel.vetter@ffwll.ch