drm: omapdrm: Move encoder setup to encoder operations

Now that the driver is fully converted to atomic operations, and that
the atomic helpers call the operations in the right order, we can move
encoder setup to where it belongs, in the encoder operations.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>