drm/msm/dp: use eld_mutex to protect access to connector->eld

Reading access to connector->eld can happen at the same time the
drm_edid_to_eld() updates the data. Take the newly added eld_mutex in
order to protect connector->eld from concurrent access.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241206-drm-connector-eld-mutex-v2-7-c9bce1ee8bea@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>