drm/qxl: implement prime kmap/kunmap

Generic fbdev emulation needs this.  Also: We must keep track of the
number of mappings now, so we don't unmap early in case two users want a
kmap of the same bo.  Add a sanity check to destroy callback to make
sure kmap/kunmap is balanced.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190118122020.27596-17-kraxel@redhat.com