Add tests for KMS atomic modesetting, to exercise the basic interface
and test failure/corner cases. Should ensure coherency between the
legacy and atomic interfaces.

v2: New patch.
v3: Disable connector checking for now, as it was causing GPU hangs on
    newer kernels.
v4: Rebase.
v5: Use do_ioctl or do_ioctl_err consistently. Use igt_assert_*()
    helper macros rather than igt_assert() directly.
    Move assertions into helper/check functions. Define atomic commit
    helper.
v6: Use do_ioctl_err, and define macros to move errors to
    actual callsite, rather than helper functions.

Co-authored-by: Micah Fedke <micah.fedke@collabora.com>
Signed-off-by: Daniel Stone <daniels@collabora.com>