venus: workaround a blob_mem mappable size check issue

venus: workaround a blob_mem mappable size check issue

For blob_mem allocated from virtgpu_virgl backend, the guest mappable
size queried can be smaller than the size returned from image memory
requirement query from the host side. Here we temporarily workaround
until we switch to use cross-domain backend in minigbm.

Cc: 21.2.3 mesa-stable
Signed-off-by: Yiwei Zhang <zzyiwei@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Ryan Neph <ryanneph@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12687>