drm: mxsfb: Remove unused macros from mxsfb_regs.h

mxsfb_regs.h defines macros related to register bits. Some of them are
not used and don't clearly map to any particular register, so their
purpose isn't known. Remove them.

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/20200727020654.8231-5-laurent.pinchart@ideasonboard.com