drm: move drm_mode_legacy_fb_format to drm_fourcc.c

It's part of the drm fourcc handling code, mapping the old depth/bpp
values to new fourcc codes.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829082757.17913-6-daniel.vetter@ffwll.ch