drm: omapdrm: Move size unit features to dispc_features structure

The buffer_size_unit and burst_size_unit are dispc features. Move them
from the omap_dss_features structure to the dispc_features structure.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>