qxl: check for kmap failures

qxl: check for kmap failures

BugLink: http://bugs.launchpad.net/bugs/1629386
commit f4cceb2affcd1285d4ce498089e8a79f4cd2fa66 upstream.

If kmap fails, it leads to memory corruption.

Fixes: f64122c1f6ad ('drm: add new QXL driver. (v1.4)')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160711084633.GA31411@mwanda
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>