lib/kms: Add igt_display_commit2()

lib/kms: Add igt_display_commit2()

Add a new commit interface, igt_display_commit2(), that allows tests to
specify which programming API should be used to perform hardware
updates.  COMMIT_LEGACY is the only option for now, but universal
and atomic interfaces will be added as additional options in the future.

igt_display_commit() remains unchanged for existing tests that wish to
place the hardware in a specific state, but that don't care which API is
used to achieve that state.  The legacy API will be used by default for
now, but in the future we may decide to make the default API
configurable via an environment variable.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>