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Now the scaler is enabled for the sunxi disp layer only when we want
to use it for YUV format with XV. Whenever the layer is configured
for RGB format or deactivated, the scaler gets disabled.

This should make the driver more friendly to the other potential
scaled layer users. The total number of available scalers is only
2 for Allwinner A10 and only 1 for Allwinner A13.

The potential drawback is that now we may get an error when trying
to enable the scaler (if somebody else has used up all the available
scalers) instead of always having it reserved and ready for use.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>