OMAPDSS: DISPC: Don't pass channel out when configuring overlays

dispc_ovl_setup_common() is to be used by both overlays and writeback. We pass
channel out to figure out what manager the overlay is connected to, to determine
the pixel clock rate. This is used to decide the scaling limitations for that
overlay.

writeback doesn't have a channel out, it has a channel in field which tells
where writeback gets its input from. These are 2 different fields, and this
prevents us reusing the overlay configuration code for writeback.

To overcome this, we now pass omap_plane to overlay related functions rather
than passing channel out. We create helper functions which can derive pclk/lclk
from the omap_plane id.
Signed-off-by: Archit Taneja <archit@ti.com>