drm/i915/guc: Propagate the fw xfer timeout

Propagate the timeout on transferring the fw back to the caller where it
may act upon it, usually by restarting the xfer before failing.

v2: Simplify the wait to only wait upon the guc signaling completion,
with an assertion that the fw xfer must have completed for it to be
ready!

Testcase: igt/drv_selftest/live_hangcheck
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181018195536.11522-1-chris@chris-wilson.co.uk