Add a `GpuManager::device_mut()` method #4801

Job	Run time
format	11s
clippy-check	2m 48s
smithay-check-features	5m 0s
smallvil-check	50s
check-minimal	1m 9s
anvil-check-features	2m 12s
check-msrv	1m 4s
Documentation on Github Pages	1m 10s
smithay-tests	2m 27s
WLCS: Core tests	1m 14s
WLCS: Output tests	1m 16s
WLCS: Pointer input tests	1m 11s
WLCS: Bad Buffer Test	1m 21s
	21m 53s

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