lavfi/gradfun: remove check for AV_PERM_PRESERVE.

This check does not make sense in this context, see
doc/filter_design.txt for details about the usage of such flag.

libavfilter/vf_gradfun.c

@@ -189,7 +189,7 @@ static int filter_frame(AVFilterLink *inlink, AVFilterBufferRef *in)
     AVFilterBufferRef *out;
     int p, direct = 0;
 
-    if ((in->perms & AV_PERM_WRITE) && !(in->perms & AV_PERM_PRESERVE)) {
+    if (in->perms & AV_PERM_WRITE) {
         direct = 1;
         out = in;
     } else {