main/linux-lts: enable HDMI LPE driver
I know that 3.16 is soon, and this module is already enabled in the edge LTS kernel (so presumably 3.16 will have this already enabled too?), but it would be useful for me if it was also available on 3.15 in the meantime.
I only enabled it for x86{,_64} because, as far as I can tell, this is an x86 thing.