blast: fix dropped 32 bits of layer_state_t::what

layer_state_t::what is 64 bits so the corresponding bit mask
should also be 64 bits

Test: Open chrome, play video, put chrome in the background and
      bring it back to the foreground. Make sure the image looks
      correct.
Bug: N/A

Change-Id: I5c414f5d3eceaaff74d3a7d62467b5838f87af97