drm/arc: Use drmm_mode_config_cleanup

drm/arc: Use drmm_mode_config_cleanup

With autocleanup through drm_device management we can delete all the
code. Possible now that there's no confusion against devm_kzalloc'ed
structures anymore.

I inlined arcpgu_setup_mode_config because it's tiny and the newly
needed return value handling would have been more ...

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112084358.2771527-7-daniel.vetter@ffwll.ch