radeonsi: Move NULL check before dereference.

radeonsi: Move NULL check before dereference.

Fix defect reported by Coverity Scan.

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

Fixes: 2f83dce059 ("radeonsi: don't report R64_*INT as a sampler format because it doesn't work")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16380>