drm/vc4: bo: Rename vc4_dumb_create

We're going to add a new variant of the dumb BO allocation function, so
let's rename vc4_dumb_create() to something a bit more specific.

Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220610115149.964394-4-maxime@cerno.tech