drm/i915: Adjust watermark register masks

We want to be able to use the masks to decode the register contents
regardless of the hardware generation. So just expand the masks to
cover all available bits, even if those are reserved on some
generations.

v2: Don't extend WM1_LP_SR_MASK so far, for the *future*

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>