OMAPDSS: get dssdev->alias from DT alias

We currently create a "displayX" style alias name for all displays,
using a number that is incremented for each registered display. With the
new DSS device model there is no clear order in which the displays are
registered, and thus the numbering is somewhat random.

This patch improves the behavior for DT case so that if the displays
have been assigned DT aliases, those aliases will be used as DSS
aliases.

This means that "display0" is always the one specified in the DT alias,
and thus display0 can be used as default display in case the user didn't
specify a default display.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>