drm/omap: Major omap_modeset_init() cleanup

Cleanup overly complex omap_modeset_init(). The function is trying to
support many unusual configuration, that have never been tested and
are not supported by other parts of the dirver.

After cleanup the init function creates exactly one connector,
encoder, crtc, and primary plane per each connected dss-device. Each
connector->encoder->crtc chain is expected to be separate and each
crtc is connect to a single dss-channel. If the configuration does not
match the expectations or exceeds the available resources, the
configuration is rejected.

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