Avoid ueventd CPU spinning at 60hz for vsync

If you strace the ueventd process when the screen is on you see it is spinning on VSYNC= update events. It uses about 7% CPU, surfaceflinger possibly does too and other system processes that watch for uevents. This patch fixes that, tested on a mango phone.

The newer method is to use /sys/devices/virtual/graphics/fb0/vsync_event instead - see https://github.com/LegacyXperia/android_hardware_qcom_display-legacy/commit/ff4f0254be575a264504687c407e0db2fd5573d7

The code was already there but got reverted in https://github.com/LegacyXperia/msm7x30-3.4.x-nAa/commit/07e06c43d3cb67df84b37d58026a7ee3bc9e0019 - I guess that was unintentional with all the other fixes there.