drm/vc4: bo: Split out Dumb buffers fixup

The vc4_bo_dumb_create() both fixes up the allocation arguments to match
the hardware constraints and actually performs the allocation.

Since we're going to introduce a new function that uses a different
allocator, let's split the arguments fixup to a separate function we
will be able to reuse.

Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220610115149.964394-5-maxime@cerno.tech