drm/omap: Add dispc_mgr_get_supported_outputs()

We are removing the use of the 'struct omap_overlay_manager' from
omapdrm, and one part of that is removing the use of
mgr->supported_outputs field.

This patch adds dispc_mgr_get_supported_outputs() function which can be
used instead of mgr->supported_outputs. omap_crtc.c is changed to use
the new function.

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