drm/i915/guc: Introduce guc_is_ready

We already have guc_is_running function, but it only reflects
firmware status, while to fully use GuC we need to know if we've
already established communication with it.

v2: also s/intel_guc_is_running/intel_guc_is_fw_running (Chris)

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/20200131153706.109528-1-michal.wajdeczko@intel.com