drm/vkms: Use drmm_add_final_kfree

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

v2: After drm_dev_init/drmm_add_final_kfree we need to clean up
everything through a drm_dev_put. Rework the unwind code to match
that.

Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Tested-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-15-daniel.vetter@ffwll.ch