drm/msm/dp: handle PHY directly in dp_ctrl

There is little point in going trough dp_parser->io indirection each
time the driver needs to access the PHY. Store the pointer directly in
dp_ctrl_private.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Tested-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/576119/
Link: https://lore.kernel.org/r/20240126-dp-power-parser-cleanup-v3-11-098d5f581dd3@linaro.org