radv: Remove unnecessary null check.

radv: Remove unnecessary null check.

Fix defect reported by Coverity Scan.

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

Fixes: 779e09639b5 ("radv: configure DB_Z_INFO.NUM_SAMPLES correctly on GFX11")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17100>