drm/rockchip: Include <drm/drm_gem_cma_helper> for drm_gem_cm_vm_ops

Include <drm/drm_gem_cma_helper.h> to get drm_gem_cma_vm_ops. Fallout
from the recent conversion to GEM object functions.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 0d590af3 ("drm/rockchip: Convert to drm_gem_object_funcs")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: "Heiko Stübner" <heiko@sntech.de>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200928081643.8575-1-tzimmermann@suse.de