drm/omap: cleanup formats array

omap_fb.c has a table with DSS and DRM formats, used to convert between
them. This is no longer needed, so we can change the array to a plain
array of DRM_FORMAT_* values which contain all possible pixel formats
supported by any DSS IP version.

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