OMAPDSS: rename omap_dss_device's 'device' field to 'dst'

In the old panel device model we had omap_dss_output entities,
representing the encoders in the DSS block. This entity had "device"
field, which pointed to the panel that was using the omap_dss_output.

With the new panel device model, the omap_dss_output is integrated into
omap_dss_device, which now represents a "display entity". Thus the "device"
field, now in omap_dss_device, points to the next entity in the display
entity-chain.

This patch renames the "device" field to "dst", which much better tells
what the field points to.

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