drm/qxl: add NULL check for bo->resource

drm/qxl: add NULL check for bo->resource

When allocations fails that can be NULL now.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Tested-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Tested-by: Roberto Sassu <roberto.sassu@huawei.com>
Fixes: 0abecef40ccc ("drm/ttm: allocate resource object instead of embedding it v2")
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708114710.8186-1-christian.koenig@amd.com