i965: Let the caller of brw_set_dp_write/read_message control the target cache.

i965: Let the caller of brw_set_dp_write/read_message control the target cache.

brw_set_dp_read_message already had a target_cache argument, but its
interpretation was rather convoluted (on Gen6 the render cache was
used if the caller asked for it, otherwise it was ignored using the
sampler cache instead), and the constant cache wasn't representable at
all.  brw_set_dp_write_message used the data cache on Gen7+ except for
RENDER_TARGET_WRITE messages, in which case it would use the render
cache.  On Gen6 the render cache was always used.

Instead of the above, provide the shared unit SFID that the caller
expects will be used.  Makes no functional changes.

v3: Non-trivial rebase.

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