drm/i915: Unconditionally clear plane visibility, v2.

We need to assume the plane has been visible before, even if no CRTC
is assigned to the plane. This is because when enabling a nv12 plane
on gen11, we will have to enable an extra plane and make it visible
by marking it in crtc_state->active_planes for
intel_update_planes_on_crtc().

Additionally, clear visible flag in intel_plane_atomic_check, in case
we ever hit a bug with visibility. Our code implicitly assumes that
plane_state->visible is only true when crtc and fb are set,
so we will either null deref in intel_fbc_choose_crtc() or
do something bad during the actual commit which cares even more.

Changes since v1:
- Unconditionally clear crtc_state->active_planes as well.
- Reword commit message, since this is now a preparation patch for
  NV12 Y / UV plane linking.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[mlankhorst: Clear nv12_planes in the beginning as well, clarify commit message]
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920102711.4184-4-maarten.lankhorst@linux.intel.com