arm: omap boards: Remove unnecessary platform_enable/disable callbacks for VENC devices

The omap_dss_device's platform_enable/disable callbacks don't do anything for
any of the boards. The platform calls from the VENC driver will also be removed
in the future. Remove these calls from the board which have a VENC device.

Signed-off-by: Archit Taneja <archit@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>