gallium/ttn: Convert to using VARYING_SLOT_* / FRAG_RESULT_*.

gallium/ttn: Convert to using VARYING_SLOT_* / FRAG_RESULT_*.

This avoids exceeding the size of the .index bitfield since it got
truncated, and should make our NIR look more like the NIR that the rest of
the NIR developers are working on.

v2: split out vc4 updates, first patch uses varying_slot_to_tgsi_semantic()
    helper, and second patch does the actual conversion.
v3: add frag_result_to_tgsi_semantic() helper and don't try to map
    frag_results to semantic name/index as if they were varying_slot's
v4: use VERT_ATTRIB_ for VS inputs
v5: Fix vc4 build.

Signed-off-by: Rob Clark <robclark@freedesktop.org>