drm/i915: Wait for gen3 reset status to be asserted

drm/i915: Wait for gen3 reset status to be asserted

After we assert the reset request (and wait for 20us), when the device
has been fully reset it asserts the reset-status bit. Before we stop
requesting the reset and allow the device to return to normal, we should
wait for the reset to be completed. (Similar to how we wait for the
device to return to normal after deasserting the reset request.)

v2: Rename i915_reset_completed() probe to not cause as much confusion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207222824.29864-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>