drm/omap: output: use dev_err instead of DSSERR

We don't have omapdss's custom error printing functions in the common
omapdss-base module, to which we want to move output.c.

This patch changes output.c to use dev_err instead of DSSERR so that it
doesn't depend on DSSERR.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>