drm/i915: add WM_LINETIME registers

Watermark line time registers for display low power watermark.

v2: improve bit names as suggested by Chris Wilson

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>