drm/i915: Fix intel_atomic_setup_scalers() plane_state handling

Since the plane_state variable is declared outside the scaler_users
loop in intel_atomic_setup_scalers(), and it's never reset back to
NULL inside the loop we may end up calling intel_atomic_setup_scaler()
with a non-NULL plane state for the pipe scaling case. That is bad
because intel_atomic_setup_scaler() determines whether we are doing
plane scaling or pipe scaling based on plane_state!=NULL. The end
result is that we may miscalculate the scaler mode for pipe scaling.

The hardware becomes somewhat upset if we end up in this situation
when scanning out a planar format on a SDR plane. We end up
programming the pipe scaler into planar mode as well, and the
result is a screenfull of garbage.

Fix the situation by making sure we pass the correct plane_state==NULL
when calculating the scaler mode for pipe scaling.

Cc: stable@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231207193441.20206-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

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