drm/omap: rename panel/encoder Kconfig names

omapdrm is using much too generic Kconfig names for its panels and
encoders. Rename them to have "DRM_OMAP" in the name.

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