drm/i915: Remove crtc->config dereference from drrs_ctl

Wait for idle, and iterate over connectors instead of encoders.
With this information we know crtc->state is the actual state,
and we can enable/disable drrs safely.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181011100457.8776-2-maarten.lankhorst@linux.intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>