qxl: check for kmap failures

If kmap fails, it leads to memory corruption.

Fixes: f64122c1 ('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