drm/i915: Move SKL IPC WA to HAS_IPC()

SKL has IPC but it should not be set according to the WA, so lets
just mark as it don't have it to simply the code and avoid
unnecessary MMIO writes at every call to intel_enable_ipc().

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/20180918204714.27306-4-jose.souza@intel.com