drm/i915: Distinguish between timeout and error in sideband transactions

drm/i915: Distinguish between timeout and error in sideband transactions

After initiating a sideband transaction, we only want to wait for the
transaction to become idle. If, as we are, we wait for both the busy
and error flag to clear, if an error is raised we just spin until the
timeout. Once the hw is idle, we can then check to see if the hw flagged
an error, and report it distinctly.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223141020.13250-1-chris@chris-wilson.co.uk
Reviewed-by: Jani Nikula <jani.nikula@intel.com>