drm: mxsfb: Remove register definitions from mxsfb_crtc.c

mxsfb_crtc.c defines several macros related to register addresses and
bit, which duplicates macros from mxsfb_regs.h. Use the macros from
mxsfb_regs.h instead and 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/20200727020654.8231-9-laurent.pinchart@ideasonboard.com