i965: Move data from brw->vs into a base class if gs will also need it.

i965: Move data from brw->vs into a base class if gs will also need it.

This paves the way for sharing the code that will set up the vertex
and geometry shader pipeline state.

v2: Rename the base class to brw_stage_state.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>