drm/qxl: remove set but not used variable 'bo_old'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/qxl/qxl_display.c: In function 'qxl_primary_atomic_update':
drivers/gpu/drm/qxl/qxl_display.c:538:17: warning:
 variable 'bo_old' set but not used [-Wunused-but-set-variable]

It's not used any more after 4979904c

 ("drm/qxl: use shadow bo directly")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190218085459.196470-1-yuehaibing@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>