drm: mxsfb: Clarify format and bus width configuration

Replace the convoluted way to set the format and bus width through
difficult to read macros with more explicit ones. Also remove the
outdated comment related to the limitations on bus width setting as it
doesn't apply anymore (the bus width can be specified through the
display_info bus format).

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/20200727020654.8231-6-laurent.pinchart@ideasonboard.com