drm/vkms: Fix possible memory leak in _vkms_get_crc()

'vaddr_out' is malloced in _vkms_get_crc() and should be freed before
leaving from the error handling cases, otherwise it will cause memory
leak.

Fixes: db7f419c ("drm/vkms: Compute CRC with Cursor Plane")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1536976399-1295-1-git-send-email-weiyongjun1@huawei.com