drm: fix division-by-zero on dumb_create()

Kinda unexpected, but DIV_ROUND_UP() can overflow if passed an argument
bigger than UINT_MAX - DIVISOR. Fix this by testing for "!cpp" before
using it in the following division.

Note that DIV_ROUND_UP() is defined as:
        #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))

..this will obviously overflow if (n + d - 1) is bigger than UINT_MAX.
Reported-by: NTommi Rantala <tt.rantala@gmail.com>
Signed-off-by: NDavid Herrmann <dh.herrmann@gmail.com>
Reviewed-by: NRob Clark <robdclark@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc.c

@@ -4696,8 +4696,9 @@ int drm_mode_create_dumb_ioctl(struct drm_device *dev,
 		return -EINVAL;
 
 	/* overflow checks for 32bit size calculations */
+	/* NOTE: DIV_ROUND_UP() can overflow */
 	cpp = DIV_ROUND_UP(args->bpp, 8);
-	if (cpp > 0xffffffffU / args->width)
+	if (!cpp || cpp > 0xffffffffU / args->width)
 		return -EINVAL;
 	stride = cpp * args->width;
 	if (args->height > 0xffffffffU / stride)