SourceLevelDebugPkg: Fix PEI debug timer regression

SourceLevelDebugPkg: Fix PEI debug timer regression

Recent changes to debug timer handling ended up leaving the timer disabled in
PEI.  This made it impossible to stop execution in PEI externally via the
debugger.  Enable the timer when InitializeDebugAgent calls
InitializeDebugAgentPhase2, as well as when it returns directly.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Brian J. Johnson <bjohnson@sgi.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17819 6f19259b-4bc3-4df7-8a09-765794883524