modetest: unify {,atomic_}set_mode()

Instead of duplicating the exact same code across the two functions,
fold them into one.

For some strange reason git diff may show atomic_clear_mode() as changed
The function in untouched, despite the misleading output.

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