i965: Don't try to apply the opt_sampler_eot extension for vs

The opt_sampler_eot optimisation of fs_visitor effectively assumes
that it is running on a fragment shader because it casts the program
key to a brw_wm_prog_key. However on Skylake fs_visitor can also be
used for vertex shaders. It looks like this usually works anyway
because the optimisation is skipped if key->nr_color_regions != 1.
However for a vertex shader the key is actually a brw_vs_prog_key so
the space for nr_color_regions is probably taken up by
key->base.program_string_id. This can end up making nr_color_regions
be 1 in which case the function will later assert when the last
instruction is not FS_OPCODE_FB_WRITE. This was making the DEQP test
suite assert. Presumably this only happens there because that compiles
a lot of shaders so it would end up with a high value for
program_string_id.

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

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -2548,6 +2548,9 @@ fs_visitor::opt_sampler_eot()
 {
    brw_wm_prog_key *key = (brw_wm_prog_key*) this->key;
 
+   if (stage != MESA_SHADER_FRAGMENT)
+      return false;
+
    if (devinfo->gen < 9 && !devinfo->is_cherryview)
       return false;