mm-video: Allow devices to override QComOMXMetadata header

* Certain manufacturers add extra fields to the
  encoder_media_buffer_type struct.

Change-Id: I2485196c2ab5d798a1dcd2353c9f8cc5114987f8
Merge pull request #1 from CyanogenMod/cm-11.0

mm-video: Allow devices to override QComOMXMetadata header