drm/vc4: hvs: Create cob_init function

drm/vc4: hvs: Create cob_init function

Just like the HVS itself, the COB parameters will be fairly different in
the BCM2712.

Let's move the COB parameters computation and its initialisation to a
separate function that will be easier to extend in the future.

Signed-off-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240621152055.4180873-25-dave.stevenson@raspberrypi.com
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>