clk: qcom: dispcc-sm8250: stop using mmcx regulator

Now as the common qcom clock controller code has been taught about power
domains, stop mentioning mmcx supply as a way to power up the clock
controller's gdsc.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>