OMAPDSS: Remove omap_panel_config enum from omap_dss_device

omap_panel_config contains fields which are finally written to DISPC_POL_FREQo
registers. These are now held by omap_video_timings and are set when the manager
timings are applied.

Remove the omap_panel_config enum, and remove all it's references from panel or
interface drivers.

Signed-off-by: Archit Taneja <archit@ti.com>