[rbp/omxplayer] Ignore speaker layout for analogue

Although speaker configuration options are only visible for hdmi,
the code still uses them after switching back to analogue
resulting in bad audio.
[rbp] Avoid crash when useddsfanart is enabled

Pi textures don't contain pixels on the arm, and the dds encode path wasn't protected against that.
Also don't allow dds to be enabled, as it is undesirable on Pi