drm/msm/dsi: properly handle the case of empty OPP table in dsi_mgr_bridge_mode_valid

It was left unnoticed during the review that even if there is no OPP
table in device tree, one will be created by a call to the function
devm_pm_opp_set_clkname(). This leads to dsi_mgr_bridge_mode_valid()
rejecting all modes if DT contains no OPP table for the DSI host.

Rework dsi_mgr_bridge_mode_valid() to handle this case by actually
checking that the table is populated with frequency entries before
returning an error.

Fixes: 8328041b

 ("drm/msm/dsi: implement opp table based check for dsi_mgr_bridge_mode_valid()")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/520076/
Link: https://lore.kernel.org/r/20230124203600.3488766-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>