video: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200221160005.GA13552@embeddedor

drivers/video/fbdev/mmp/hw/mmp_ctrl.h

@@ -1406,7 +1406,7 @@ struct mmphw_ctrl {
 
 	/*pathes*/
 	int path_num;
-	struct mmphw_path_plat path_plats[0];
+	struct mmphw_path_plat path_plats[];
 };
 
 static inline int overlay_is_vid(struct mmp_overlay *overlay)

drivers/video/fbdev/ssd1307fb.c

@@ -89,7 +89,7 @@ struct ssd1307fb_par {
 
 struct ssd1307fb_array {
 	u8	type;
-	u8	data[0];
+	u8	data[];
 };
 
 static const struct fb_fix_screeninfo ssd1307fb_fix = {

include/video/mmp_disp.h

@@ -231,7 +231,7 @@ struct mmp_path {
 
 	/* layers */
 	int overlay_num;
-	struct mmp_overlay overlays[0];
+	struct mmp_overlay overlays[];
 };
 
 extern struct mmp_path *mmp_get_path(const char *name);