Move GrD3DDescriptorHeap::GenID out of header

The only user is the constructor so there is no need to put the code
into the header. Moving this into the cpp file avoids declaring a static
global in a header, which triggers the newer unique-object-duplication
clang warning.

Change-Id: Id72b471b3dd59f81c9d51767af9f9da9c21f13ff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1205059
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>