drm/i915: Drop unused position fields (v2)

The userspace-requested plane coordinates are now always available via
plane->state.base (and the i915-adjusted values are stored in
plane->state), so we no longer use the coordinate fields in intel_plane
and can drop them.

Also, note that the error case for pageflip calls update_plane() to
program the values from plane->state; it's simpler to just call
intel_plane_restore() which does the same thing.

v2: Replace manual update_plane() with intel_plane_restore() in pageflip
    error handler.

Reviewed-by(v1): Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>