hwcomposer: Disable MDP_BACKEND_COMPOSITION for video

 * This can cause the dreaded "pipe still staged" black screen of death.
 * Remove the flag when video is playing.

Change-Id: Ia187c6232e40abe569edef280226340bf32911c6
Merge pull request #1 from CyanogenMod/cm-10.1

hwcomposer: Disable MDP_BACKEND_COMPOSITION for video