drm/i915: Don't pass plane to .check_plane()

.check_plane() already gets the plane state, so we can dig out the plane
from there if needed. No need in passing it separately.

Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180828142707.31583-1-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>