drm/i915: Don't try to partition WOPCM without GuC firmware

For meaningful WOPCM partitioning we need GuC (and optionally HuC)
firmware size(s) and we shouldn't just rely on GuC support flag,
as we might fail to fetch GuC firmware and it's size will be 0
and all calculations will be just wrong/useless.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190807170034.8440-5-michal.wajdeczko@intel.com