Added check valid value of LookAheadDS
Added check valid value of LookAheadDS

In some cases where LookAheadDS is out of range we get MFX_ERR_NONE. For example
if LookAheadDS=4 we don't get errors because LaDSenumToFactor returns valid
value=4 as is the case MFX_LOOKAHEAD_DS_4x. But it isn't right due to
MFX_LOOKAHEAD_DS_4x=3 by enum, so we need to check LookAheadDS.

Intel-Media-SDK/MediaSDK,Intel-Media-SDK/MediaSDK,Intel-Media-SDK/MediaSDK,Intel-Media-SDK/MediaSDK,Intel-Media-SDK/MediaSDK