hwc: bring sanity to vsync loop code

* Shrink the buffer from 4k to 32 bytes; it's used to store one unsigned
  64-bit int in decimal representation, so 4k is INSANELY oversized
* Make timestamp unsigned -- the value from the kernel is unsigned and
  we're parsing it using strtoull()
* Don't read into the last byte of the buffer (to ensure
  null termination)
* Null-terminate the buffer immediately after the last read byte
* Remove unused variable

Change-Id: Idfe0d8c8f4fe57928b3acf55f70c96de010b60e6