drm/vc4: vec: Check for VEC output constraints

The VEC can accept pretty much any relatively reasonable mode, but still
has a bunch of constraints to meet.

Let's create an atomic_check() implementation that will make sure we
don't end up accepting a non-functional mode.

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Tested-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Acked-in-principle-or-something-like-that-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v10-16-256dad125326@cerno.tech


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