drm/i915/gt: document masked registers

Document what a masked register is according to bspec so we avoid
developers using the wrong functions to implement WAs.

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/20201209045246.2905675-3-lucas.demarchi@intel.com