msdk: check GstMsdkContext instead of mfxSession instance
When creating a GstMsdkContext instance, it also creates a mfxSession instance, so we may check GstMsdkContext instead of mfxSession instance to make sure MSDK is available. In addition, according to MSDK doc [1], MFXVideoCORE_SetHandle function should be executed before any actual usage of library including queries, otherwise the behavior is unexpected, so we should call MFXVideoCORE_QueryPlatform after MFXVideoCORE_SetHandle on Linux