drm/vc4: Add dependency on HAVE_DMA_ATTRS, and select DRM_GEM_CMA_HELPER

Avoid the following build errors, seen with m68k:allmodconfig and other
architectures which do not support HAVE_DMA_ATTRS.

ERROR: "drm_gem_cma_create" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_prime_mmap" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_prime_get_sg_table" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_vm_ops" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_mmap" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_prime_vunmap" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_prime_import_sg_table" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_free_object" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_prime_vmap" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_dumb_map_offset" [drivers/gpu/drm/vc4/vc4.ko] undefined!
ERROR: "drm_gem_cma_create" [drivers/gpu/drm/drm_kms_helper.ko] undefined!
ERROR: "drm_gem_cma_describe" [drivers/gpu/drm/drm_kms_helper.ko] undefined!
ERROR: "drm_gem_cma_free_object" [drivers/gpu/drm/drm_kms_helper.ko] undefined!

Acked-by: Eric Anholt <eric@anholt.net>
Fixes: c8b75bca ("drm/vc4: Add KMS support for Raspberry Pi.")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>