drm/vc4: Support nomodeset

If we have nomodeset on the kernel command line we should have the
firmware framebuffer driver kept as is and not try to load the
full-blown KMS driver.

In this case, let's just register the v3d driver.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211215095117.176435-3-maxime@cerno.tech