drm_fb_helper: Preserve capability to use atomic kms

Commit 5349ef3127c77075ff70b2014f17ae0fbcaaf199 (drm/fb: fix
FBIOGET/PUT_VSCREENINFO pixel clock handling) changed the logic of
when a pixclock was valid vs invalid.

The atomic kernel mode setting used by the kernel debugger relies upon
the drm_fb_helper_check_var() to always return -EINVAL.  Until a
better solution exists, this behavior will be restored.

CC: David Airlie <airlied@linux.ie>
CC: Jesse Barnes <jbarnes@virtuousgeek.org>
CC: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

drivers/gpu/drm/drm_fb_helper.c

@@ -685,7 +685,7 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
 	struct drm_framebuffer *fb = fb_helper->fb;
 	int depth;
 
-	if (var->pixclock != 0)
+	if (var->pixclock != 0 || in_dbg_master())
 		return -EINVAL;
 
 	/* Need to resize the fb object !!! */