drm/qxl: rename qxl_bo_kmap -> qxl_bo_vmap_locked

Append _locked to Make clear that these functions should be called with
reserved bo's only.  While being at it also rename kmap -> vmap.

No functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210217123213.2199186-6-kraxel@redhat.com