p/prober: disable all drivers when qwerty is enabled
qwerty is auto probed, making HMDs that are not auto probed always take precedence.
When setting QWERTY_ENABLE=1 the intent is usually to exclusively use qwerty.
Therefore we default to disabling all other drivers when this variable is set.
To make the old behavior of adding qwerty devices with lower priority than actual
devices, the variable QWERTY_COMBINE=1 is introduced.
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