OMAPDSS: DISPC: use dss_feat_get_num_ovls()

Use dss_feat_get_num_ovls() in dispc.c instead of
omap_dss_get_num_overlays() to remove the dependency to overlay.c. Note
that we still have uses of omap_dss_get_num_overlays() in dispc.c, but
these will be moved out in the future patches.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>