drm/i915/guc: Avoid wasting memory on incorrect GuC pin bias

It would appear that the calculated GuC pin bias was larger than it should
be, as the GuC address space does NOT contain the "HW contexts RSVD" part
of the WOPCM. Thus, the GuC pin bias is simply the GuC WOPCM size.

v5:
Clarify the diagram to better represent the GuC address space.
Since we now don't use guc.base for the pin bias there's no need to
validate it. It also has already been verified in WOPCM init.

Bspec: 1180

Signed-off-by: Jakub Bartmiński <jakub.bartminski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180727141148.30874-1-jakub.bartminski@intel.com