OMAPDSS: DSI: delay dispc initialization

We currently setup both DSI and DISPC related things when the DSI bus is
enabled. There's no need for DISPC related thing at that point, though,
but only later when the video output is enabled.

To make it possible to use the DSI bus before DISPC overlay manager is
selected, this patch moves DSI's DISPC initialization to
dsi_enable_video_output(), from omapdss_dsi_display_enable(). We also
move the selection of DISPC's LCD clock to dsi_enable_video_output.

This way there are no DISPC dependencies until the video output is
enabled.

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