SF: Fix incorrect early sf gl phase information dumped from SurfaceFlinger

early sf gl phase is equal to debug.sf.early_gl_phase_offset_ns if this
property is set, or it is equal to sf phase.

Also add ATRACE_TAG for DispSyncSource

Test: Check early sf gl is correct in dumpsys SurfaceFlinger
Test: VSYNC-app and VSYNC-sf shows on systrace

Change-Id: I9f5797c2e6e731d44c7521e8c37cefb9d3602d06