i965: Combine render target resolve code

i965: Combine render target resolve code

We have two different bits of resolve code for render targets: one in
brw_draw where it's always been and one in brw_context to deal with sRGB
on gen9.  Let's pull them together.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>