gstomxh264enc: make the buffer writable before passing it to the encoder

gst_buffer_ref in the line can affect the buffer's writability,
so make sure its writable afterwards.

previously, with the following reproducer, there's a critical about the
buffer's writability every ~2.5 seconds on a rpi3b+.

```
$ gst-launch-1.0 videotestsrc ! video/x-raw,width=1280,height=720,framerate=25/1 ! glupload ! glcolorconvert ! gldownload ! omxh264enc ! video/x-h264,profile=high ! fakesink
..
(gst-launch-1.0:26155): GStreamer-CRITICAL **: 10:18:15.497: gst_buffer_add_meta: assertion 'gst_buffer_is_writable (buffer)' failed
..
```
12 jobs for !76 with alatiera/omx-writable in 6 minutes and 26 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Build Docker
passed alpine amd64 manifest builder docker #9025124

00:00:33

passed android docker #9025123

00:03:45

passed cerbero fedora amd64 docker #9025122

00:04:08

passed fedora amd64 docker #9025121

00:06:17

passed gst-indent amd64 docker #9025125

00:00:28

passed windows amd64 docker #9025126
1809 windows shell

00:00:23

 
  Preparation
passed check allow-edit #9025129

00:00:09

passed gst indent #9025128

00:00:08

manual manifest #9025127
manual
 
  Build
created build gst-omx tizonia fedora x86_64 #9025131
created build gst-omx zynq fedora x86_64 #9025130
 
  Integrate
created documentation #9025132