intel_prepare_plane_fb() will always pin plane_state->hw.fb whenever
it is present. We copy that from the master plane to the slave plane,
but we fail to copy the corresponding ggtt view. Thus when it comes time
to pin the slave plane's fb we use some stale ggtt view left over from
the last time the plane was used as a non-slave plane. If that previous
use involved 90/270 degree rotation or remapping we'll try to shuffle
the pages of the new fb around accordingingly. However the new
fb may be backed by a bo with less pages than what the ggtt view
rotation/remapped info requires, and so we we trip a GEM_BUG().

Steps to reproduce on icl:
1. plane 1: whatever
   plane 6: largish !NV12 fb + 90 degree rotation
2. plane 1: smallish NV12 fb
   plane 6: make invisible so it gets slaved to plane 1
3. GEM_BUG()

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/intel/issues/951
Fixes: 1f594b20 ("drm/i915: Remove special case slave handling during hw programming, v3.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200110183228.8199-1-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>