drm: Make the prime vmap/vunmap hooks optional.

Some drivers leave these unimplemented, so don't make them have
unimplemented stubs.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180424004610.4637-2-eric@anholt.net
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>