drm/i915: Fix 915GM self-refresh enable/disable

INSTPM is a masked register so use the _MASKED_BIT_{ENABLE,DISABLE}
macros when enabling/disabling self-refresh on 915GM.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>