drm/qxl: fix smatch warnings

drivers/gpu/drm/qxl/qxl_display.c:99 qxl_alloc_client_monitors_config() error: dereferencing freed memory 'qdev->client_monitors_config'
drivers/gpu/drm/qxl/qxl_object.c:66 qxl_ttm_placement_from_domain() warn: bitwise AND condition is false here
drivers/gpu/drm/qxl/qxl_ioctl.c:353 qxl_clientcap_ioctl() warn: buffer overflow 'qdev->rom->client_capabilities' 58 <= 58

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>