drm/i915: Move sprite/cursor plane disable to intel_sanitize_crtc()

Move the sprite/cursor plane disabling to occur in intel_sanitize_crtc()
where it belongs instead of doing it in intel_modeset_readout_hw_state().

The plane disabling was first added in
4cf0ebbd

 drm/i915: Rework plane readout.

I got the idea from some patches from Partik and/or Maarten but those
moved also the plane state readout to intel_sanitize_crtc() which isn't
quite right in my opinion.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=91910
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[Jani: cherry-picked from -next to v4.3]
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>