drm/i915: Drop unpin stall in atomic_prepare_commit

The core already does this in setup_commit(). With this we can also
remove the unpin_work_count since it's the last user, and also remove
the loop since that was only used for stalling against legacy flips.

v2: Amend commit message a bit (Chris).

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/20170720175754.30751-8-daniel.vetter@ffwll.ch