drm/i915: Remove vma unpin in intel_plane_destroy

drm/i915: Remove vma unpin in intel_plane_destroy

commit aede17c43632ab8da4f81b79ac79c54cc7fd125c
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Thu Dec 15 15:29:44 2016 +0100

    drm/i915: Disable all crtcs during driver unload, v2.

made sure that all crtc's are disabled on driver unload, but only the
following commit made sure all fb's are cleaned up correctly:

commit b275868ed32932501a35bd909ae0cc7092d253b8
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Tue Feb 21 14:51:40 2017 +0100

    drm/atomic: Make disable_all helper fully disable the crtc.

Finally remove this and add a WARN_ON when vma is set. It should
have been removed by intel_cleanup_plane_fb().

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511082844.13965-2-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>