i965/fs: Combine the fs_visitor constructors.

i965/fs: Combine the fs_visitor constructors.

For scalar GS support, we either need to add a fourth constructor which
takes the GS structures, or combine the existing two and pass the shader
stage.

Given that they're not significantly different, I opted for the latter.

v2: Remove more stuff from the .h file (Jason and Jordan).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>