drm: mxsfb: Pass mxsfb_drm_private pointer to mxsfb_reset_block()

The mxsfb_reset_block() function isn't special, pass it the
mxsfb_drm_private pointer instead of a pointer to the base address.

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/20200727020654.8231-7-laurent.pinchart@ideasonboard.com