drm/i915: Make intel_crtc_disable_planes() use active planes mask.

This will only disable planes we actually had marked as visible in
crtc_state->visible_planes and cleans up intel_crtc_disable_plane()
slightly.

This is also useful for when we start enabling NV12 support for gen11,
in which we will make the separate Y plane visible, but ignore the
Y plane's state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920102711.4184-5-maarten.lankhorst@linux.intel.com