drm/fb: fix FBIOGET/PUT_VSCREENINFO pixel clock handling

When the framebuffer driver does not publish detailed timing information
for the current video mode, the correct value for the pixclock field is
zero, not -1.

Since pixclock is actually unsigned, the value -1 would be interpreted
as 4294967295 picoseconds (i.e., about 4 milliseconds) by
register_framebuffer() and userspace programs.

This patch allows X.org's fbdev driver to work.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>
Tested-by: NPaulius Zaleckas <paulius.zaleckas@gmail.com>
Cc: stable@kernel.org
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fb_helper.c

@@ -599,7 +599,7 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
 	struct drm_framebuffer *fb = fb_helper->fb;
 	int depth;
 
-	if (var->pixclock == -1 || !var->pixclock)
+	if (var->pixclock != 0)
 		return -EINVAL;
 
 	/* Need to resize the fb object !!! */
@@ -691,7 +691,7 @@ int drm_fb_helper_set_par(struct fb_info *info)
 	int ret;
 	int i;
 
-	if (var->pixclock != -1) {
+	if (var->pixclock != 0) {
 		DRM_ERROR("PIXEL CLCOK SET\n");
 		return -EINVAL;
 	}
@@ -904,7 +904,7 @@ int drm_fb_helper_single_fb_probe(struct drm_device *dev,
 	fb_helper->fb = fb;
 
 	if (new_fb) {
-		info->var.pixclock = -1;
+		info->var.pixclock = 0;
 		if (register_framebuffer(info) < 0)
 			return -EINVAL;
 	} else {