drm/i915: Band Gap WA

Note: No one seems to have docs for this, so this patch here is just
unreviewed black magic :(

Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: ymohanma <yogesh.mohan.marimuthu@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Add note about the doc situation.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>