drm/vboxvideo: Use drm_gem_vram_vmap() interfaces

VRAM helpers support ref counting for pin and vmap operations, no need
to avoid these operations by employing the internal kmap interface. Just
use drm_gem_vram_vmap() and let it handle the details.

Also unexport the kmap interfaces from VRAM helpers. Vboxvideo was the
last user of these internal functions.

v2:
	* fixed a comma in commit description

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200911075922.19317-1-tzimmermann@suse.de