drm: rcar-du: crtc: Make local functions static

The rcar_du_crtc_mode_valid() and rcar_du_crtc_get_crc_sources()
functions are accessed only through a function pointer table.

Convert the function definitions to be static to the module.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>