Make gdk_visual_get_*_pixel_details work again

These functions are supposed to return the numbers of consecutive
1 bits in each components mask as precision. However, due to a
copy-paste mistake when this code was moved around in
commit 70d689cddd, the precision
was always reported as zero. This affects only a few applications
that directly set window background on X11 windows, such as emacs.

https://bugzilla.gnome.org/show_bug.cgi?id=764210

gdk/gdkvisual.c

@@ -371,7 +371,6 @@ gdk_visual_get_pixel_details (GdkVisual *visual,
           m >>= 1;
         }
 
-      m = pixel_mask;
       while (m & 0x1)
         {
           p++;