Most of the helpers to retrieve vc4 structures from the DRM base structures
rely on the fact that the first member of the vc4 structure is the DRM one
and just cast the pointers between them.

However, this is pretty fragile especially since there's no check to make
sure that the DRM structure is indeed at the offset 0 in the structure, so
let's use container_of to make it more robust.
Signed-off-by: NMaxime Ripard <maxime@cerno.tech>
Reviewed-by: NDave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028123752.1733242-1-maxime@cerno.tech