drm/i915/uc: Use GuC firmware status helper

We already have helper function for checking GuC firmware
load status. Replace existing open-coded checks.

v2: drop redundant USES_GUC check

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@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/20190522193203.23932-7-michal.wajdeczko@intel.com