Mark switch cases where we are expecting to fall through.

Fix the following warnings (Building: mpc512x_defconfig powerpc):

drivers/video/fbdev/fsl-diu-fb.c: In function ‘fsl_diu_ioctl’:
./include/linux/device.h:1750:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  _dev_warn(dev, dev_fmt(fmt), ##__VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/video/fbdev/fsl-diu-fb.c:1287:3: note: in expansion of macro ‘dev_warn’
   dev_warn(info->dev,
   ^~~~~~~~
drivers/video/fbdev/fsl-diu-fb.c:1290:2: note: here
  case MFB_SET_PIXFMT:
  ^~~~
In file included from ./include/linux/acpi.h:15:0,
                 from ./include/linux/i2c.h:13,
                 from ./include/uapi/linux/fb.h:6,
                 from ./include/linux/fb.h:6,
                 from drivers/video/fbdev/fsl-diu-fb.c:20:
./include/linux/device.h:1750:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  _dev_warn(dev, dev_fmt(fmt), ##__VA_ARGS__)
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/video/fbdev/fsl-diu-fb.c:1296:3: note: in expansion of macro ‘dev_warn’
   dev_warn(info->dev,
   ^~~~~~~~
drivers/video/fbdev/fsl-diu-fb.c:1299:2: note: here
  case MFB_GET_PIXFMT:
  ^~~~
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Timur Tabi <timur@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911113604.GA31512@embeddedor