drm/modes: Skip invalid cmdline mode

The named mode could be invalid and then cmdline parser misses to validate
mode's dimensions, happily adding 0x0 mode as a valid mode. One case where
this happens is NVIDIA Tegra devices that are using downstream bootloader
which adds "video=tegrafb" to the kernel's cmdline and thus upstream Tegra
DRM driver fails to probe because of the invalid mode.

Fixes: 3aeeb13d89 ("drm/modes: Support modes names on the command line")
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190709145151.23086-1-digetx@gmail.com

drivers/gpu/drm/drm_client_modeset.c

@@ -180,6 +180,7 @@ again:
 
 create_mode:
 	mode = drm_mode_create_from_cmdline_mode(connector->dev, cmdline_mode);
+	if (mode)
 		list_add(&mode->head, &connector->modes);
 
 	return mode;

drivers/gpu/drm/drm_modes.c

@@ -158,6 +158,9 @@ struct drm_display_mode *drm_cvt_mode(struct drm_device *dev, int hdisplay,
 	int interlace;
 	u64 tmp;
 
+	if (!hdisplay || !vdisplay)
+		return NULL;
+
 	/* allocate the drm_display_mode structure. If failure, we will
 	 * return directly
 	 */
@@ -392,6 +395,9 @@ drm_gtf_mode_complex(struct drm_device *dev, int hdisplay, int vdisplay,
 	int hsync, hfront_porch, vodd_front_porch_lines;
 	unsigned int tmp1, tmp2;
 
+	if (!hdisplay || !vdisplay)
+		return NULL;
+
 	drm_mode = drm_mode_create(dev);
 	if (!drm_mode)
 		return NULL;