freedreno/a3xx: fix texture buffers, enable offsets

freedreno/a3xx: fix texture buffers, enable offsets

The main issue is that the current logic looked into cso->u.tex, which
is the wrong side of the union to look into for texture buffers. While I
was at it, it was easy enough to add the logic to handle offsets
(first_element).

 - reduce texture buffer size limit (determined experimentally)
 - don't look at first/last levels, instead look at first/last element
 - include the first element offset
 - set offset alignment to 16 (determined experimentally)

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Rob Clark <robclark@freedesktop.org>