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

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

drivers/gpu/drm/qxl/qxl_object.c: In function 'qxl_bo_kunmap_atomic_page':
drivers/gpu/drm/qxl/qxl_object.c:189:21: warning:
 variable 'map' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1541821486-40631-1-git-send-email-yuehaibing@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>