v4l2: increase pre-allocated encoded buffer size
v4l2: increase pre-allocated encoded buffer size

As of today, the MFC encoder often need to exceed that 1 MB
size for encoded buffer we fixed earlier for decoding.

https://bugzilla.gnome.org/show_bug.cgi?id=728438

GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good