drm: mxsfb: Use BIT() macro to define register bitfields

Using BIT() is preferred over manual shifts as it's more readable,
handles the 1 << 31 case properly, and avoids other mistakes as shown by
the DEBUG0_HSYNC and DEBUG0_VSYNC bits (that are currently unused). Use
it.

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/20200727020654.8231-4-laurent.pinchart@ideasonboard.com