drm/rcar-du: Use the GEM PRIME helpers

The GEM CMA PRIME import/export helpers have been removed in favor of
generic GEM PRIME helpers with GEM CMA low-level operations. Fix the
driver accordingly.

Reported-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Mark Brown <broonie@linaro.org>