asahi: Move assignment after null check.

Fix defect reported by Coverity Scan.

Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking so suggests that it may be null,
but it has already been dereferenced on all paths leading to the
check.

Fixes: dcd2d8ca500 ("asahi: Track more Gallium state")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11898>
1 file changed