[AVR32] Drop GFP_COMP for DMA memory allocations

dma_alloc_coherent wants to split pages after allocation in order to
reduce the memory footprint. This does not work well with GFP_COMP
pages, so drop this flag before allocation.

This patch was forward-ported from BSP 2.0

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>