i965/blorp: Refactor surface format determination.

i965/blorp: Refactor surface format determination.

This patch moves the responsibility for deciding on the format of the
source and destination surfaces from the
gen{6,7}_blorp_emit_surface_state() functions to
brw_blorp_surface_info::set(), which is shared between Gen6 and Gen7.
This will make it possible to add support for more surface formats
without code duplication.

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