i965/meta: initialize values to avoid random behaviour on error path

i965/meta: initialize values to avoid random behaviour on error path

if brw_meta_stencil_blit() errored at wrong place 'target' would
be uninitialized and cause random behaviour on leaving the funtion.

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>