i965: Fix = vs == in MCS aux usage assert.

Caught by Coverity (CID 1415680).

Cc: "17.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

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

@@ -1103,7 +1103,7 @@ brw_blorp_mcs_partial_resolve(struct brw_context *brw,
    DBG("%s to mt %p layers %u-%u\n", __FUNCTION__, mt,
        start_layer, start_layer + num_layers - 1);
 
-   assert(mt->aux_usage = ISL_AUX_USAGE_MCS);
+   assert(mt->aux_usage == ISL_AUX_USAGE_MCS);
 
    const mesa_format format = _mesa_get_srgb_format_linear(mt->format);
    enum isl_format isl_format = brw_blorp_to_isl_format(brw, format, true);