drm: omapdrm: dpi: Remove platform driver

The DPI platform driver was used for non-DT platforms only. On DT
platforms the DPI port is handled manually. As OMAP display devices are
now instantiated from DT only, remove the DPI platform driver.

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>