drm/vc4: kms: Split the HVS muxing check in a separate function

The code that assigns HVS channels during atomic_check is starting to grow
a bit big, let's move it into a separate function.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Reviewed-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105135656.383350-5-maxime@cerno.tech