drm/i915: Add a FIXME about FBC vs. fence. 90/270 degree rotation

drm/i915: Add a FIXME about FBC vs. fence. 90/270 degree rotation

Currently the FBC code doesn't handle the 90/270 degree rotated case
correctly. We would need the GTT tracking to monitor the fence on the
normal GTT view (the rotated view doesn't even have a fence). Not quite
sure how we should program the fence Y offset etc. in that case. For now
we'll end up disabling FBC with 90/270 degree rotation. Add a FIXME
to remind people about this fact.

v2: Reword the text (Chris)
    Move the FIXME to the fbc code

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221160235.11134-7-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>