drm/i915: Add wa_ctx_emit_reg()

Add a helper for emitting register offsets (for LRI/SRM) into the w/a
batch buffer.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446672017-24497-21-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>