i965: Generalize wm_key->compute_sample_id to wm_key->multisample_fbo.

I'm going to need a key entry meaning "we have a multisample FBO,
and multisampling is enabled" in an upcoming patch.  This is basically
wm_key->compute_sample_id, except that it also checks that the SAMPLE_ID
system value is read.

The only use of wm_key->compute_sample_id is in emit_sampleid_setup(),
which is only called when handling the SAMPLE_ID system value.  So we
can just eliminate the check and generalize the field.

v2: Also update the Vulkan driver.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/intel/vulkan/anv_pipeline.c

@@ -288,7 +288,7 @@ populate_wm_prog_key(const struct brw_device_info *devinfo,
        */
       key->persample_shading = info->pMultisampleState->sampleShadingEnable;
       key->compute_pos_offset = info->pMultisampleState->sampleShadingEnable;
-      key->compute_sample_id = info->pMultisampleState->sampleShadingEnable;
+      key->multisample_fbo = true;
    }
 }