fbdevhw: Use displayWidth for fbdev virtual width when appropriate.

The fbdev API doesn't allow setting the pitch explicitly, so we have to set
the virtual width to the pitch we're using for drawing. This fixes corruption
after changing the virtual width with RandR.
(cherry picked from commit d077c0d)

hw/xfree86/fbdevhw/fbdevhw.c

@@ -183,7 +183,8 @@ print_xfree_mode(char *txt, DisplayModePtr mode)
 static void
 xfree2fbdev_fblayout(ScrnInfoPtr pScrn, struct fb_var_screeninfo *var)
 {
-	var->xres_virtual   = pScrn->virtualX;
+	var->xres_virtual   = pScrn->displayWidth ? pScrn->displayWidth :
+			      pScrn->virtualX;
 	var->yres_virtual   = pScrn->virtualY;
 	var->bits_per_pixel = pScrn->bitsPerPixel;
 	var->red.length     = pScrn->weight.red;