drm/xe: Use a define to set initial seqno for fences

Also for HW fences, write the initial seqno - 1 to the HW completed
seqno to initialize.

v2:
- Use __dma_fence_is_later() to compare hw fence seqnos. (Matthew Auld)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>