drm/mediatek/dp: Fix spurious kfree()

drm_edid_to_sad() might return an error or just zero. If that is the
case, we must not free the SADs because there was no allocation in
the first place.

Fixes: dab12fa8 ("drm/mediatek/dp: fix memory leak on ->get_edid callback audio detection")
Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20240604083337.1879188-1-mwalle@kernel.org/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>