drm: mxsfb: Use LCDC_CTRL register name explicitly

The LCDC_CTRL register is located at address 0x0000. Some of the
accesses to the register simply use the mxsfb->base address. Reference
the LCDC_CTRL register explicitly instead to clarify the code.

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/20200727020654.8231-8-laurent.pinchart@ideasonboard.com