drm/i915: Do not modifiy reserved bit in gens that do not have IPC

IPC was only added in SKL+(actually we don't even enable for SKL due
WA) so without this change, driver was writing to a reserved bit.

Also removing the uncessary dev_priv->ipc_enabled = false; as now
gens without IPC will not have IPC enabled.

v2(Rodrigo):
- moved the new handling of WA #0477 to the next patch

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/20180918204714.27306-3-jose.souza@intel.com