drm/vc4: plane: Take possible_crtcs as an argument

vc4_plane_init() currently initialises the plane with no possible CRTCs,
and will expect the caller to set it up by itself.

Let's change that logic a bit to follow the syntax of
drm_universal_plane_init() and pass the possible CRTCs bitmask as an
argument to the function instead.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>