drm/vc4: Introduce generation number enum

With the introduction of the BCM2712 support, we will get yet another
generation of display engine to support.

The binary check of whether it's VC5 or not thus doesn't work anymore,
especially since some parts of the driver will have changed with BCM2711,
and some others with BCM2712.

Let's introduce an enum to store the generation the driver is running
on, which should provide more flexibility.

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