drm/i915/uc: Consider enable_guc modparam during fw selection

We can use value of enable_guc modparam during firmware path selection
and start using firmware status to see if GuC/HuC is being used.
This is first step to make enable_guc modparam read-only.

v2: rebased, don't care about <0 (Chris)
v3: oops

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/20190731223321.36436-3-michal.wajdeczko@intel.com