r600: use GET_BUFFER_RESINFO vtx fetch on eg instead of setting up consts

r600: use GET_BUFFER_RESINFO vtx fetch on eg instead of setting up consts

Contrary to what the comment said, this appears to work just fine on my rv770
(tested with piglit textureSize 140 fs/vs samplerBuffer).
Dave Airlie confirmed it working on cayman too.
I have no clue though if it's actually preferrable to use it (unfortunately
we cannot get rid of the tex constants completely, as we still require them
for cube map txq).
Albeit filling in the format (1 channels or 4?) and the stuff related to mega-
or mini-fetch (what the hell is this...) is just a guess based on other usage
of vtx fetch instructions...

v2: it really needs to be done through texture cache (I botched the
testing because sb optimizations turned it automatically into tc, but
can't rely on it and isn't happening on tes).

Tested-by: Konstantin Kharlamov <hi-angel@yandex.ru>
Reviewed-by: Dave Airlie <airlied@redhat.com>