drm/qxl: Replace deprecated function in qxl_display

Replace deprecated function drm_modeset_lock/unlock_all with
helper function DRM_MODESET_LOCK_ALL_BEGIN/END.

Signed-off-by: Sidong Yang <realwakka@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200524022624.10363-1-realwakka@gmail.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>