Do not allow transient/invalid display state targets

Under certain circumstances mce will not honor display state requests
that are made. Choosing altogether different target state causes no
harm, but an attempt to keep existing state while in transitional
state causes illogical target state to be assigned and logic failures
in the underlying state machine.

Now requests that invalid from the state machine point of view are
ignored, which effectively keeps the display state unchanged.

[mce] Do not allow transient/invalid display state targets. Fixes JB#21994
Merge pull request #206 from spiiroin/display_request_filtering

Do not allow transient/invalid display state targets