mmc: meson-gx: avoid clock glitch when switching to DDR modes

Activating DDR in the Amlogic mmc controller, among other things, will
divide the output clock by 2. So by activating it with clock on, we are
creating a glitch on the output.

Instead, let's deal with DDR when the clock output is off, when setting
the clock.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>