drm/i915: Decouple struct i915_params i915 into i915_params.h

Otherwise usage in the i915 debug macros yields problems due to
i915_drv.h <-> i915_trace.h <-> intel_drv.h include loops.

v2:
- Document not-so-obvious need for linux/cache.h (Chris)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450436898-20408-2-git-send-email-joonas.lahtinen@linux.intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>