drm/i915: Cancel reset preparations on failed resets

Our reset handling has a retry layer further up in the
chain. As we have told the engine to prepare for reset,
and failed it, make sure to remove that preparation so
that the next attempted reset has a clean slate by triggering
another full prepare cycle for the engines.

v2: ret as int, simplified cleanup (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180605160357.32591-1-mika.kuoppala@linux.intel.com