drm/i915: Parametrize and fix SWF registers

Parametrize the SWF registers. This also fixes the register offsets,
which were mostly garbage in the old defines.

Also save/restore only as many SWF registers that each platform has.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>