avcodec/cbs_mpeg2: use memcpy when assembling fragments

before:
419022 decicycles in assemble_fragment,    2047 runs,      1 skips

after:
104621 decicycles in assemble_fragment,    2045 runs,      3 skips

Benched with a 2 minutes long 720x480 DVD mpeg2 sample.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/cbs_mpeg2.c

@@ -362,7 +362,7 @@ static int cbs_mpeg2_assemble_fragment(CodedBitstreamContext *ctx,
                                        CodedBitstreamFragment *frag)
 {
     uint8_t *data;
-    size_t size, dp, sp;
+    size_t size, dp;
     int i;
 
     size = 0;
@@ -382,8 +382,8 @@ static int cbs_mpeg2_assemble_fragment(CodedBitstreamContext *ctx,
         data[dp++] = 0;
         data[dp++] = 1;
 
-        for (sp = 0; sp < unit->data_size; sp++)
-            data[dp++] = unit->data[sp];
+        memcpy(data + dp, unit->data, unit->data_size);
+        dp += unit->data_size;
     }
 
     av_assert0(dp == size);