clk: qcom: dispcc-sm8250: Add RETAIN_FF_ENABLE flag for mdss_gdsc

All SoC supported by this driver supports the RETAIN_FF_ENABLE flag,
so it should be enabled here.

This feature enables registers to maintain their state after
dis/re-enabling the GDSC.

Signed-off-by: Robert Foss <robert.foss@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221102090140.965450-3-robert.foss@linaro.org