radeonsi: do not set the shader->key for gs copy shaders

radeonsi: do not set the shader->key for gs copy shaders

The key for a geometry shader would be interpreted as the key for a vertex
shader further down the line, which really doesn't make sense.

This does not affect the contents of shader->key because geometry shaders
don't have any key entries anyway.

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>