drm: Include internal header for managed function declarations

drivers/gpu/drm/drm_managed.c:61:6: warning: symbol 'drm_managed_release' was not declared. Should it be static?
  CC      drivers/gpu/drm/drm_managed.o
drivers/gpu/drm/drm_managed.c:61:6: warning: no previous prototype for ‘drm_managed_release’ [-Wmissing-prototypes]
 void drm_managed_release(struct drm_device *dev)

Fixes: c6603c74

 ("drm: add managed resources tied to drm_device")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200516212330.13633-1-chris@chris-wilson.co.uk