drm/i915: Introduce bit definitions of CTXT_SR_CTRL register.

This patch introduces 2 bit definitions of context save/restore
control register.

Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Suggested-by: Dave Gordon <david.s.gordon@intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>