drm: rockchip: select DRM_GEM_CMA_HELPER

drm: rockchip: select DRM_GEM_CMA_HELPER

The rockchip drm driver started using drm_gem_cma_vm_ops, but that might
not be part of the kernel, causing the link to fail:

drivers/gpu/built-in.o:(.data+0xb234): undefined reference to `drm_gem_cma_vm_ops'

This adds a Kconfig 'select' statement to enable it like the other
user do.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 80f67cd80add ("drm/rockchip: Use cma gem vm ops")
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160616122800.1174015-1-arnd@arndb.de