platform/x86: dell-laptop: Allocate buffer on heap rather than globally

[<Mario.Limonciello@xxxxxxxx>]

commit 9862b43624 upstream

There has been a race condition identified with initialization of the dell-laptop
driver due to changes in 4.15.  This commit has been shown to resolve the race
condition.

I'd like to see this brought back to 4.15 stable.  It doesn't apply to anything earlier.

Thanks,