drm/i915: Convert 'flip_queued_seqno' into 'flip_queued_request'

Converted the flip_queued_seqno value to be a request structure as part of the
on going seqno to request changes. This includes reference counting the request
being saved away to ensure it can not be retired and freed while the flip code
is still waiting on it.

For: VIZ-4377
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Thomas Daniel <Thomas.Daniel@intel.com>
[danvet: Again get rid of the _irq request unref by simply moving that
into the unpin worker. Doesn't matter when we hang onto the request
for a bit longer, and in the unpin worker we already grab the
dev->struct_mutex anyway.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>