drm/i915: Rename hsw_lp_wm_result to intel_wm_level

Let's call hsw_lp_wm_result intel_wm_level from now on and move it to
i915_drv.h for later use.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>