drm/vc4: drv: Skip BO Backend Initialization on BCM2711

On the BCM2711, we currently call the vc4_bo_cache_init() and
vc4_gem_init() functions. These functions initialize the BO and GEM
backends.

However, this code was initially created to accomodate the requirements
of the GPU on the older SoCs, while the BCM2711 has a separate driver
for it. So let's just skip these calls when we're on a newer hardware.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>