drm/vram: Acquire lock only once per call to vmap()/vunmap()

The implementation of vmap() is a combined pin() and kmap(). As both
functions share the same lock, we can make vmap() slightly faster by
acquiring the lock only once for both operations. Same for the inverse,
vunmap().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reported-and-tested-by: Davidlohr Bueso <dbueso@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190906122056.32018-3-tzimmermann@suse.de