drm/msm/dp: Clean up pdev/dev duplication in dp_power

drm/msm/dp: Clean up pdev/dev duplication in dp_power

The dp_power module keeps track of both the DP controller's struct
platform_device and struct device - with the prior pulled out of the
dp_parser module.

Clean up the duplication by dropping the platform_device reference and
just track the passed struct device.

Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/536939/
Link: https://lore.kernel.org/r/20230515030256.300104-3-quic_bjorande@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>