drm: Move format-related helpers to drm_fourcc.c

The drm_crtc.c file is a mess, making the ABI documentation confusing
since all functions are in the same bag. Split the format-related
helpers to a new drm_fourcc.c file.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465466048-2020-1-git-send-email-laurent.pinchart@ideasonboard.com