drm/i915: remove WA_SET_FIELD_MASKED()

Remove the last macro and implement it as a function like the rest of
the operations that don't assume there is a `wal` list, but rather
receive it as argument.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201205092542.2325477-4-lucas.demarchi@intel.com