tests/modetest: Add atomic support

If "-a" option is set this make modetest use atomic API instead
of legacy API.

Test the frame rate ("-v") it does a loop and swap between two
framebuffer for each active planes.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>