DRM_GEM_CMA_HELPER is depend on HAVE_DMA_ATTRS, or it will break the
building. The related error (with allmodconfig under xtensa):

    CC [M]  drivers/gpu/drm/drm_gem_cma_helper.o
  drivers/gpu/drm/drm_gem_cma_helper.c: In function 'drm_gem_cma_create':
  drivers/gpu/drm/drm_gem_cma_helper.c:110:19: error: implicit declaration of function 'dma_alloc_writecombine' [-Werror=implicit-function-declaration]
    cma_obj->vaddr = dma_alloc_writecombine(drm->dev, size,
                     ^
  drivers/gpu/drm/drm_gem_cma_helper.c:110:17: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
    cma_obj->vaddr = dma_alloc_writecombine(drm->dev, size,
                   ^
  drivers/gpu/drm/drm_gem_cma_helper.c: In function 'drm_gem_cma_free_object':
  drivers/gpu/drm/drm_gem_cma_helper.c:193:3: error: implicit declaration of function 'dma_free_writecombine' [-Werror=implicit-function-declaration]
     dma_free_writecombine(gem_obj->dev->dev, cma_obj->base.size,
     ^
  drivers/gpu/drm/drm_gem_cma_helper.c: In function 'drm_gem_cma_mmap_obj':
  drivers/gpu/drm/drm_gem_cma_helper.c:330:8: error: implicit declaration of function 'dma_mmap_writecombine' [-Werror=implicit-function-declaration]
    ret = dma_mmap_writecombine(cma_obj->base.dev->dev, vma,
          ^
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>