drm/i915: Remove '& 0xffff' from the mask given to WA_REG()

We may be hidding bugs by doing that, so let remove it and have the
actual mask value shine through, for better or worse.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>