drm/i915: Invert the mask and val arguments in wa_add() and WA_REG()

While trying to unify the order of those arguments throughout the
driver, Daniel noticed what we were inverting them in this part of the
code.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>