drm/i915/guc: Assert that all GGTT offsets used by the GuC are mappable

Add an assertion to the plain i915_ggtt_offset() to double check that
any offset we hand to the GuC is outside of its unmappable ranges.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161224193146.4402-1-chris@chris-wilson.co.uk


Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>