drm/i915: check gtfifodbg after possibly failed writes
If we don't have a sufficient number of free entries in the FIFO, we
proceed to do a write anyway. With this check we should have a clue if
that write actually failed or not.
After some discussion with Daniel Vetter regarding his original
complaint, we agreed upon this.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> Signed-off-by: Ben Widawsky <ben@bwidawsk.net> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>