i965/fs: Don't consider the stencil output to be a color output.

This would cause gl_FragStencilRef to be counted as a color output
incorrectly during the precompile phase, which leads to unnecessary
recompilation on master and could trigger an assertion failure in
fs_visitor::emit_fb_writes() on my i965-fb-fetch branch.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_wm.c

@@ -608,7 +608,8 @@ brw_fs_precompile(struct gl_context *ctx,
 
    key.nr_color_regions = _mesa_bitcount_64(fp->Base.OutputsWritten &
          ~(BITFIELD64_BIT(FRAG_RESULT_DEPTH) |
-         BITFIELD64_BIT(FRAG_RESULT_SAMPLE_MASK)));
+           BITFIELD64_BIT(FRAG_RESULT_STENCIL) |
+           BITFIELD64_BIT(FRAG_RESULT_SAMPLE_MASK)));
 
    key.program_string_id = bfp->id;