OMAPDSS: rename omap_dss_device's 'output' to 'src'

In the old panel device model we had "outputs", which were the encoders
inside OMAP DSS block, and panel devices (omap_dss_device). The panel
devices had a reference to the source of the video data, i.e. reference
to an "output", in a field named "output".

That was somewhat confusing even in the old panel device model, but even
more so with the panel device model where we can have longer chains of
display entities.

This patch renames the "output" field to "src", 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>