intel: Add common/intel_genX_state.h, replace brw_fs_get_dispatch_enables() with genX intel_update_ps_state()
First intel_genX_state.h is added. This file is intended to have "genX" functions that the various drivers can use when emitting state.
Then, intel_update_ps_state() is added to intel_genX_state.h in order to replace brw_fs_get_dispatch_enables() from @llandwerlin's b9403b1c. Instead of returning the dispatch enables as 3 separate bools, intel_update_ps_state() updates GENX(3DSTATE_PS).
We expect the interface to brw_fs_get_dispatch_enables() will not map well to future platforms, and @currojerez suggested that perhaps we could add a function to change GENX(3DSTATE_PS) more directly.