drm: mxsfb: Turn mxsfb_set_pixel_fmt() into a void function

The mxsfb_set_pixel_fmt() function returns an error when the selected
pixel format is unsupported. This can never happen, as such errors are
caught by the DRM core. Remove the error check.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-20-laurent.pinchart@ideasonboard.com