st/pbo: fix pbo uploads without PIPE_CAP_TGSI_VS_LAYER_VIEWPORT

st/pbo: fix pbo uploads without PIPE_CAP_TGSI_VS_LAYER_VIEWPORT

the code here tries to be too smart and only use a geometry shader if there's
actually multiple layers being uploaded, but the fragment shader also unconditionally
reads gl_Layer as long as the pipe cap for gs is set, which means that
in the case when the gs is dynamically disabled due to uploading a
single-layer surface, the fs has no input to read for gl_Layer and everything breaks

always using a gs isn't ideal, but it's considerably more work to manage multiple
fs variants based on layer usage

Fixes: c99f2fe70ec ("st/mesa: implement PBO upload for multiple layers")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8067>