drm/i915: Shut up GuC errors when it's disabled

drm/i915: Shut up GuC errors when it's disabled

DRM_ERROR an continue without any issues aren't allowed since that
causes noise in the CI system. But we absolutely want to have the
DRM_ERROR when we want to run with GuC.

For simplicity just short-circuit all the loader code when it's not
needed.

v2: Mika&Chris complained that I shouldn't hit send on patches written
before coffee kicks in.

v3: Make it compile at least ...

Cc: Alex Dai <yu.dai@intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445591459-4327-1-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>