i965/fs: Guess nr_color_regions better in precompile

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

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

@@ -3590,7 +3590,9 @@ brw_fs_precompile(struct gl_context *ctx, struct gl_shader_program *prog)
       key.render_to_fbo = _mesa_is_user_fbo(ctx->DrawBuffer);
    }
 
-   key.nr_color_regions = 1;
+   key.nr_color_regions = _mesa_bitcount_64(fp->Base.OutputsWritten &
+         ~(BITFIELD64_BIT(FRAG_RESULT_DEPTH) |
+         BITFIELD64_BIT(FRAG_RESULT_SAMPLE_MASK)));
 
    /* GL_FRAGMENT_SHADER_DERIVATIVE_HINT is almost always GL_DONT_CARE.  The
     * quality of the derivatives is likely to be determined by the driconf