i965: Silence brw_blorp uninitialized warning

The variables level and start_layer are not initialized, then
initialized if we have a BUFFER_BIT_DEPTH set.  We assert on them
later using the same check.  This should be enough but GCC 9.1.1 is
not convinced, so let's initialize the variables.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

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

@@ -1405,7 +1405,7 @@ brw_blorp_clear_depth_stencil(struct brw_context *brw,
    if (x0 == x1 || y0 == y1)
       return;
 
-   uint32_t level, start_layer, num_layers;
+   uint32_t level = 0, start_layer = 0, num_layers;
    struct blorp_surf depth_surf, stencil_surf;
 
    struct intel_mipmap_tree *depth_mt = NULL;