fix deadlock in AtomicDrmDevice

[m4rch3n1ng]

88cdf9b introduced a deadlock when creating AtomicDrmDevice, as

smithay/src/backend/drm/device/atomic.rs

Line 122 in 88cdf9b

let mut mapping = dev.prop_mapping.write().unwrap();

causes a deadlock in

smithay/src/backend/drm/device/atomic.rs

Line 185 in 88cdf9b

let prop_mapping = self.prop_mapping.read().unwrap();

, since the mapping is dropped too late. this changes it to be dropped earlier.

[Drakulix]

Thanks!