drm/i915: introduce struct intel_set_config

intel_crtc_set_config is an unwidly beast and is in serious need of
some function extraction. To facilitate that, introduce a struct to
keep track of all the state involved. Atm it doesn't do much more than
keep track of all the allocated memory.

v2: Apply some bikeshed to intel_set_config_free, as suggested by
Jesse Barnes.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>