drm/omap: move dispc related dss-feat funcs to dispc

omapdrm still uses a few non-dispc functions: dss_feat_get_num_mgrs(),
dss_feat_get_num_ovls() and  dss_feat_get_supported_color_modes(). We
want to provide omapdrm a single dispc_ops function pointer struct so
that omapdrm will use either the current omapdss or the new omapdss6
driver depending on the platform.

Those three functions are really dispc functions, but are located in the
dss_features.c for legacy reasons.

This patch adds similar functionss to the dispc, and changes omapdrm to
use those new functions. Underneath the functions still call the
functions from dss_feature.c.

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