drm/i915: Remove plane_config from struct intel_crtc, v2.

Nothing depends on this outside initial hw readout, so keep this
struct on the stack instead.

Changes since v1:
- Remove unrelated changes.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>