drm/i915: re-use computed offset bias for context pin

drm/i915: re-use computed offset bias for context pin

The context has to obey the same offset requirements as the ring,
so we can re-use the same bias value we computed for the ring instead of
unconditionally using GUC_WOPCM_TOP.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1482537382-28584-2-git-send-email-daniele.ceraolospurio@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>