drm/gem: Export drm_gem_vmap() and drm_gem_vunmap()

The symbols will be required by the upcoming helpers for shadow-buffered
planes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210208115538.6430-3-tzimmermann@suse.de