This is used way too often in the enable/disable paths. And will
be even more useful in the future.

Note that correct semantics of this change highly depend upon
correct updating of intel_crtc->config: Like with all other
modeset state, we need to call ->disable with the old config,
but ->mode_set and ->enable with the new config.

v2: Do not yet use the flag in the ->disable callbacks - atm we don't
yet have support for the information stored in the pipe_config in the
hw state readout code, so this will be wrong at boot-up/resume.

v3: Rebased on top of the hdmi/dp ddi encoder merging.

v4: Fixup stupid rebase error which lead to a NULL vfunc deref.

v5: On haswell the VGA port is on the PCH!

v6: s/IS_HASWELL/HAS_DDI/, spotted by Paulo Zanoni. Also add a missing
parameter name in a function declaration.

v7: Don't forget to git add ...
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>