video: fbdev: mb862xx: remove set but not used variable 'mdr'

Fix the following gcc warning:

drivers/video/fbdev/mb862xx/mb862xxfb_accel.c:187:6: warning: variable
‘mdr’ set but not used [-Wunused-but-set-variable]
  int mdr;
      ^~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403022553.33320-1-yanaijie@huawei.com