drm: omapdrm: Don't forward set_min_bus_tput() to no-op platform code

The OMAP implementation of the set_min_bus_tput() API is a no-op.
There's no point in forwarding the driver calls to the platform code.
Remove the use of the related platform data callback, but keep the
internal function as a reminder that the feature will need to be
implemented when the OMAP platform will provide support.

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