i965/blorp: Implement proper texel fetch messages for Gen7.

i965/blorp: Implement proper texel fetch messages for Gen7.

On Gen6, texel fetch is always accomplished using the SAMPLE_LD
message, which accepts arguments (u, v, r, lod, si).  On Gen7, there
are two* texel fetch messages: SAMPLE_LD for non-MSAA surfaces, taking
arguments (u, lod, v), and SAMPLE_LD2DSS for MSAA surfaces, taking
arguments (si, u, v).

*Technically, there are other texel fetch messages, but they are used
for "compressed" MSAA surfaces, which we don't yet support.

This patch adds the proper message types and argument orderings for
Gen7.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>