drm: Remove drmm_add_final_kfree() declaration from public headers

The drmm_add_final_kfree() function is declared in the
include/drm/drm_managed.h public header, but has become an internal API
not exposed to drivers. Drop it from drm_managed.h as it's already
declared in drm_internal.h.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204092932.21636-1-laurent.pinchart+renesas@ideasonboard.com