radeonsi: silence a Coverity warning

radeonsi: silence a Coverity warning

Coverity doesn't understand that we'll never pass non-NULL for vertex
shaders.

This is a bit lame, actually. A straightforward cross-procedural analysis
limited to this source file should be enough to prove that there's no
NULL-pointer dereference. Oh well.

CID: 1405999
Reviewed-by: Marek Olšák <marek.olsak@amd.com>