CMake: libav/ffmpeg cleanups

Use @Orphis's FindFFmpeg module from ppsspp:
https://github.com/hrydgard/ppsspp/commit/2149d3db7fb960d140bdee31a9f0f4837934127b

From that commit:

> This new module should be able to handle both libraries in the regular
> paths and fallback to pkg-config.
> It is also able to find dynamic libraries, not just static libraries.
> It will generate imported targets with the name FFmpeg::<lib> that you
> can use in your scripts.

Source/Core/VideoCommon/CMakeLists.txt

@@ -62,12 +62,14 @@ else()
   set(SRCS ${SRCS} TextureDecoder_Generic.cpp)
 endif()
 
-if(LIBAV_FOUND OR WIN32)
-  set(SRCS ${SRCS} AVIDump.cpp)
-endif()
-
 add_dolphin_library(videocommon "${SRCS}" "${LIBS}")
 
-if(LIBAV_FOUND)
-  target_link_libraries(videocommon PRIVATE ${LIBS} ${LIBAV_LIBRARIES})
+if(FFmpeg_FOUND)
+  target_sources(videocommon PRIVATE AVIDump.cpp)
+  target_link_libraries(videocommon PRIVATE
+    FFmpeg::avcodec
+    FFmpeg::avformat
+    FFmpeg::avutil
+    FFmpeg::swscale
+  )
 endif()