drm/arc: Switch to devm_drm_dev_alloc

- Need to embedded the drm_device, but for now we keep the usual
  pointer chasing.
- No more devm_kzalloc, which fixes a lifetime issues on driver
  remove.
- No more drm_dev_put, that's done by devm_ now.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112084358.2771527-1-daniel.vetter@ffwll.ch