drm/v3d: Use drmm_add_final_kfree

With this we can drop the final kfree from the release function.

I also noticed that the unwind code is wrong, after drm_dev_init the
drm_device owns the v3d allocation, so the kfree(v3d) is a double-free.
Reorder the setup to fix this issue.

After a bit more prep in drivers and drm core v3d should be able to
switch over to devm_drm_dev_init, which should clean this up further.

Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-11-daniel.vetter@ffwll.ch