video: vt8500lcdfb: fix fallthrough warning

video: vt8500lcdfb: fix fallthrough warning

Fix following warning:
vt8500lcdfb.c: In function 'vt8500lcd_blank':
vt8500lcdfb.c:229:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
      if (info->fix.visual == FB_VISUAL_PSEUDOCOLOR ||
         ^
vt8500lcdfb.c:233:2: note: here
     case FB_BLANK_UNBLANK:
     ^~~~

Adding a simple "fallthrough;" fixed the warning.
The fix was build tested.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reported-by: kbuild test robot <lkp@intel.com>
Fixes: e41f1a989408 ("fbdev: Implement simple blanking in pseudocolor modes for vt8500lcdfb")
Cc: Alexey Charkov <alchark@gmail.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: <stable@vger.kernel.org> # v2.6.38+
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200412202143.GA26948@ravnborg.org