drm/i915: Make ips_enabled a property depending on whether IPS is enabled, v3.

ips_enabled was used as a variable of whether IPS can be enabled or not,
but should be used to test whether IPS is actually enabled.

Changes since v1:
- Call needs_modeset on new crtc state. (Ville)
- IPS can be enabled with sprite plane enabled too. (Ville)
- Fix CDCLK vs IPS workaround. (Ville)
Changes since v2:
- Only re-enable fastset when inheriting mode. (Ville)
- Put the conditions for enabling and disabling IPS in a helper.
Changes since v3:
- Keep the max_cdclk workaround working. (Ville)
- Also check logical cdclk out of paranoia.
- Remove planes check from IPS disable function for initial disable.
- Remove assert_plane_enabled/disabled checks and use
  crtc_state->active_planes for hsw_enable_ips only, always allow
  calling hsw_disable_ips to disable it initially in hw.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171122183901.47720-1-maarten.lankhorst@linux.intel.com
[mlankhorst: pipe_config -> crtc_state (Ville)]
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>