drm/i915: Move IPC WA #1141 to init_ipc()

symmetric_memory do not change after initialization so lets just set
ipc_enabled once for this WA.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180918204714.27306-5-jose.souza@intel.com