i965: Add a fs_visitor constructor that takes a brw_gs_compile.

i965: Add a fs_visitor constructor that takes a brw_gs_compile.

Unlike the vs/wm structs, brw_gs_compile is actually useful: it contains
the input VUE map and information about the control data headers.
Passing this in allows us to share that code in brw_gs.c, and calculate
them before deciding on vec4 vs. scalar mode, as it's independent of
that choice.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>