drm/omap: remove omap_framebuffer_get_formats()

We now get a fourcc array from dispc when asking for a plane's supported
pixel formats, so we can drop omap_framebuffer_get_formats() which was
used to convert between DSS and DRM pixel formats.

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