drm/cma-helper: Add DRM_GEM_CMA_VMAP_DRIVER_OPS

This adds functionality to the CMA helper which ensures that the kernel
virtual address is set on the CMA GEM object also for imported buffers.

The drivers have been audited to ensure that none set ->vaddr on imported
buffers, making the conditional dma_buf_vunmap() call in
drm_gem_cma_free_object() safe.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181110145647.17580-5-noralf@tronnes.org