drm/i915/guc: Fill preempt context once at init time

drm/i915/guc: Fill preempt context once at init time

Since we're inhibiting context save of preempt context, we're no longer
tracking the position of HEAD/TAIL. With GuC, we're adding a new
breadcrumb for each preemption, which means that the HW will do more and
more breadcrumb writes. Eventually the ring is filled, and we're
submitting the preemption context with HEAD==TAIL==0, which won't result
in breadcrumb write, but will trigger hangcheck instead.
Instead of writing a new preempt breadcrumb for each preemption, let's
just fill the ring once at init time (which also saves a couple of
instructions in the tasklet).

v2: Assert that context save restore is inhibited, don't assert on ring
    alignment. (Chris)
v3: Cleanup checkpatch.

Fixes: 517aaffe0c1b ("drm/i915/execlists: Inhibit context save/restore for the fake preempt context")
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180226163800.21745-1-michal.winiarski@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>