lavfi: add ff_parse_pixel_format() internal function, and use it

Reduce code duplication.

libavfilter/internal.h

@@ -63,6 +63,16 @@ int ff_fmt_is_in(int fmt, const int *fmts);
 
 /* Functions to parse audio format arguments */
 
+/**
+ * Parse a pixel format.
+ *
+ * @param ret pixel format pointer to where the value should be written
+ * @param arg string to parse
+ * @param log_ctx log context
+ * @return 0 in case of success, a negative AVERROR code on error
+ */
+int ff_parse_pixel_format(enum PixelFormat *ret, const char *arg, void *log_ctx);
+
 /**
  * Parse a sample rate.
  *