msdk: check GstMsdkContext instead of mfxSession instance

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

[1] https://github.com/Intel-Media-SDK/MediaSDK/blob/master/doc/mediasdk-man.md#working-with-va-api-applications

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1867>