drm/v3d: Rename the fence signaled from IRQs to "irq_fence".

We have another thing called the "done fence" that tracks when the
scheduler considers the job done, and having the shared name was
confusing.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190313235211.28995-2-eric@anholt.net
Reviewed-by: Dave Emett <david.emett@broadcom.com>