video: fbdev: Fix fall-through warning for Clang

Fix the following fallthrough warning (arm64-randconfig with Clang):

drivers/video/fbdev/xilinxfb.c:244:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60edca25.k00ut905IFBjPyt5%25lkp@intel.com/Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/video/fbdev/xilinxfb.c

@@ -241,6 +241,8 @@ xilinx_fb_blank(int blank_mode, struct fb_info *fbi)
 	case FB_BLANK_POWERDOWN:
 		/* turn off panel */
 		xilinx_fb_out32(drvdata, REG_CTRL, 0);
+		break;
+
 	default:
 		break;
 	}