drm/i915/guc: Make sure vma containing firmware is GuC mappable

Since commit 4741da92 ("drm/i915/guc: Assert that all GGTT offsets used
by the GuC are mappable"), we're asserting that GuC firmware is in the
GuC mappable range.
Except we're not pinning the object with bias, which means it's possible
to trigger this assert. Let's add a proper bias.

Fixes: 4741da92 ("drm/i915/guc: Assert that all GGTT offsets used by the GuC are mappable")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170111151739.28965-1-michal.winiarski@intel.com