build: exclude _studio/mfx_lib/encode on cmake level in no SW fallback build
build: exclude _studio/mfx_lib/encode on cmake level in no SW fallback build

In case of -DMFX_ENABLE_SW_FALLBACK=OFF we don't actually need to build
anything in _studio/mfx_lib/encode. So - disabling it.

Signed-off-by: Dmitry Rogozhkin <01f0df9cc32cbf6a7f0faa5c8376e6cf28354d70@intel.com>

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