drm/vc4: Prevent load tracking from breaking FKMS

Firmware KMS uses a mixture of VC4 processing and dedicated code. The
load tracking support in VC4 assumes it is dealing with vc4_plane_state
objects when up-casting with container_of, but FKMS uses unadorned
drm_plane_state structures causing the VC4 code to read off the end
into random portions of memory. Work around the problem in a minimally-
invasive way by over-allocating the FKMS plane state structures to be
large enough to contain a vc4_plane_state, filling the remainder with
zeroes.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>