modetest: factor out atomic pageflip test

Move the hunk of code into a function, making the overall flow easier to
follow and providing some symmetry to the non-atomic path.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Ezequiel Garcia <ezequiel@collabora.com>