drm/i915: Rename ilk_check_wm to ilk_validate_wm_level

Makes the behaviour of the function more clear.

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