drm/i915: adjust has_pending_fb_unpin to atomic

A bit an oversight - the current code did nothing, since only
legacy flips used the unpin_work_count and assorted logic.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170720175754.30751-6-daniel.vetter@ffwll.ch