OMAPDSS: DSI: fix dsi_get_dsidev_from_id()

If dsi_get_dsidev_from_id() is called with a DSI module id that does not
exist on the board, the function will crash as omap_dss_get_output()
will return NULL.

This happens on omap3 boards when dumping DSI clocks, as the dumping
code will try to get the dsidev for DSI modules 0 and 1, but omap3 only
has DSI module 0.

Also clean up the id -> output mapping, so that if the function is
called with invalid module ID it will return NULL.

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