drm/vc4: crtc: Switch to of_device_get_match_data

of_device_get_match_data allow to simplify a bit the retrieval of the data
associated to the pixelvalve compatible. Let's use it.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/1ff06413a1350d28bc3e88b034ed7ad23834e5bd.1590594512.git-series.maxime@cerno.tech