drm/i915: Enable IPS with only sprite plane visible too, v4.

drm/i915: Enable IPS with only sprite plane visible too, v4.

This comment predates atomic, and I think with the way we currently
track IPS, it's safe to enable this for the case we switch too.

Changes since v1:
- Keep IPS enabled when switching planes.
Changes since v2:
- Enable IPS when at least one plane is enabled. (Ville)
Changes since v3:
- Actually do what was advertised in v3, sigh! (Ville, CI)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171122183906.47767-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>