asahi: Consider shared resources always valid

asahi: Consider shared resources always valid

Shared resources are in an unknown state, so we have to assume they
may contain valid data.

Fixes Xorg+Kwin random full-screen garbage on mouse cursor present.

However, we still get black flashing in some circumstances (that was
garbage flashing before this commit), which indicates that something
is reading an uninitialized/fresh shared framebuffer and expecting
valid data (prior screen contents?) TBD.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20365>