drm/i915: request ring to be pinned above GUC_WOPCM_TOP

GuC will validate the ring offset and fail if it is in the
[0, GUC_WOPCM_TOP) range. The bias is conditionally applied only
if GuC loading is enabled (we can't check for guc submission enabled as
in other cases because HuC loading requires this fix).

Note that the default context is processed before enable_guc_loading is
sanitized, so we might still apply the bias to its ring even if it is
not needed.

v2: compute the value during ctx init and pass it to
    intel_ring_pin (Chris), updated commit message

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1482537382-28584-1-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>