avcenc: correct the description for the mode parameter
avcenc: correct the description for the mode parameter

Signed-off-by: Hai Lan <c39d70705e4b9e52aa2c8b5db2ca404f5a05ebd5@intel.com>

android-ia/hardware_intel_libva,CyanogenMod/android_hardware_intel_libva,halleyzhao/libva-fdo,android-ia/hardware_intel_libva,halleyzhao/libva-fdo,CyanogenMod/android_hardware_intel_libva,android-ia/hardware_intel_libva,CyanogenMod/android_hardware_intel_libva,halleyzhao/libva-fdo