i965/blorp: Generalize sampling code in preparation for Gen7

i965/blorp: Generalize sampling code in preparation for Gen7

This patch generalizes the function
brw_blorp_blit_program::texture_lookup() so that it prepares the
arguments to the sampler message based on a caller-provided array
rather than assuming the argument order is always (u, v).

This paves the way for the messages we will need to use in Gen7, which
use argument orders (u, lod, v) and (si, u, v) (si=sample index).

It will also will allow us to read from arbitrary sample indices on
Gen6, by supplying the arguments (u, v, r, lod, si) to the SAMPLE_LD
message instead of just (u, v).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>