drm/i915: properly mask and or watermark values for sprites

Now that we're using the sprite WM fields, we need to take care not to
clobber them in the main update_wm functions.  While we're at it, make
sure we mask out the old sprite wm value before or'ing in the new one
when the sprite wm is updated.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>