drm/i915: Cleanup properly if the implicit fence setup fails

drm/i915: Cleanup properly if the implicit fence setup fails

We've already pinned the vma and fence by the time we try to
deal with implicit fencing. Properly unpin the vma and fence
if the fence setup fails instead of just bailing straight out
from .prepare_fb(). As can be expected
drm_atomic_helper_prepare_planes() will not call .cleanup_fb()
for the plane whose .prepare_fb() failed so we must do the
cleanup ourself.

v2: Rebase

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200110183228.8199-6-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>