drm/vc4: kms: Rename NUM_CHANNELS

drm/vc4: kms: Rename NUM_CHANNELS

The NUM_CHANNELS define has a pretty generic name and was right before the
function using it. Let's move to something that makes the hardware-specific
nature more obvious, and to a more appropriate place.

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/20201105135656.383350-4-maxime@cerno.tech