omapfb: dss: Handle return errors in dss_init_ports()

dss_init_ports() is not handling return errors from dpi_init_port() and
sdi_init_port(). It is also always returning 0 currently which results in
part of error handling code in dss_bind() being unused.

Fix dss_init_ports() to handle return errors from dpi_init_port() and
sdi_init_port().

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: tomi.valkeinen@ti.com
[b.zolnierkie: fail early on errors, minor fixups]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>