dma_alloc_coherent does not return a physical address, but a DMA address,
which might be remapped or have an offset. Passing the DMA address to
vm_iomap_memory is thus broken.
Use the proper dma_mmap_coherent helper instead, and stop passing
__GFP_COMP to dma_alloc_coherent, as the memory management inside the
DMA allocator is hidden from the callers and does not require it.
With this the gfp_t argument to __videobuf_dc_alloc can be removed and
hard coded to GFP_KERNEL.