msm8974: Enable QCNE if available

Change-Id: I98858a0ef14dfdf5529355ad4f0376799e46f21f
msm8974: Clean up prebuilts

 * Add correct files for Widevine L3
 * Disable PPPreference, it can't be used with partial update
 * Remove secure display, we don't support it
 * Purge deprecated board flags

Change-Id: I42f016ac52db47777a3feeb4aa5e7170877bd93e