Build failure caused by missing EXPORT_SYMBOL_GPL(bcm_dma_wait_idle); in dma.c

Fails to build

    ERROR: "bcm_dma_wait_idle" [drivers/video/bcm2708_fb.ko] undefined!
    make[1]: *** [__modpost] Error 1
    make: *** [modules] Error 2

Tracing it through, it is because ```EXPORT_SYMBOL_GPL(bcm_dma_wait_idle);``` is missing, and shuffled ```EXPORT_SYMBOL_GPL(bcm_dma_start);``` underneath correct method.