drm/i915: Write RING_TAIL once per-request

Ignoring the legacy DRI1 code, and a couple of special cases (to be
discussed later), all access to the ring is mediated through requests.
The first write to a ring will grab a seqno and mark the ring as having
an outstanding_lazy_request. Either through explicitly adding a request
after an execbuffer or through an implicit wait (either by the CPU or by
a semaphore), that sequence of writes will be terminated with a request.
So we can ellide all the intervening writes to the tail register and
send the entire command stream to the GPU at once. This will reduce the
number of *serialising* writes to the tail register by a factor or 3-5
times (depending upon architecture and number of workarounds, context
switches, etc involved). This becomes even more noticeable when the
register write is overloaded with a number of debugging tools. The
astute reader will wonder if it is then possible to overflow the ring
with a single command. It is not. When we start a command sequence to
the ring, we check for available space and issue a wait in case we have
not. The ring wait will in this case be forced to flush the outstanding
register write and then poll the ACTHD for sufficient space to continue.

The exception to the rule where everything is inside a request are a few
initialisation cases where we may want to write GPU commands via the CS
before userspace wakes up and page flips.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>