drm/cma-helper: Remove empty drm_gem_cma_prime_vunmap()

The function drm_gem_cma_prime_vunmap() is empty. Remove it before
changing the interface to use struct drm_buf_map.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201103093015.1063-3-tzimmermann@suse.de