Defensive changes to Mac RCTDisplayLink

Real-world data suggests crashes with this stack:
CVDisplayLink::start()
-[RCTDisplayLink updateJSDisplayLinkState] (RCTDisplayLink.m:157)
-[RCTDisplayLink registerModuleForFrameUpdates:withModuleData:]_block_invoke (RCTDisplayLink.m:67)
-[RCTTiming timerDidFire] (RCTTiming.mm:324)
-[_RCTTimingProxy timerDidFire] (RCTTiming.mm:93)

Some symbols are missing in this stack, presumably due to compiler optimizations.
-updateJSDisplayLinkState is calling CVDisplayLinkStart as a result of a
call to "_jsDisplayLink.paused = NO".
-registerModuleForFrameUpdates block is presumably getting called via pauseCallback,
likely via [RCTTiming startTimers], presumably owned by RCTCxxBridge.

The most likely immediate explanation for the crash is that we are calling
CVDisplayLinkStart with a zombie _displayLink pointer.
However there is a lot of indirection here as well as thread-hopping, and
unfortunately no clearly incorrect code that would explain such a zombie pointer.

Some defensive changes:
-explicitly remove the association to pauseCallback when underlying display link object is invalidated.
-remove a prior attempt at additional check in updateJSDisplayLinkState itself as it is not relevant.
-make sure we explicitly set _displayLink to NULL when we release it, such that there is one less failure point.

⚠️ Make sure you are targeting microsoft/react-native-macos for your PR ⚠️

(then delete these lines)

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

Changelog

[CATEGORY] [TYPE] - Message

Test Plan

This was performed with the following command
git cherry-pick e877ebfe083ffaa252738477098322504492f4be