drm/i915: introduce struct intel_crtc_config

Currently only containing the requested and the adjusted mode. And
only crtc callbacks are converted somewhat to it, encoders will be
done on a as-needed basis (simply too much churn in one patch
otherwise).

Future patches will add tons more useful stuff to this struct,
starting with the very simple.

v2: Store the pipe_config in the intel_crtc, so that the ->mode-set,
->enable and also ->disable have easy access to it.

v3: Store the pipe config in the right crtc ...

v4: Rebased.

v5: Fixup an OOPS when trying to kfree an ERR_PTR.

v6: Used drm_moode_copy and some other small cleanups as suggested
by Ville Syrjälä.

v7: drm_mode_copy preserves the mode id of the destination, so no need
to clear it again (Ville).

v8: Break a long line spotted by Paulo.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>