panfrost: Fix general purpose varying handling

panfrost: Fix general purpose varying handling

When both the fragment and vertex shaders point to the same varying
location they expect to share the same varying slot.
Make sure vertex and fragment varyings pointing to the same loc have
->src_offset set to the same value.

[Alyssa: In addition a patch implement txs, this fixes GALLIUM_HUD on
Panfrost]

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>