drm/i915: Only allocate preempt context when required

If we remove some hardcoded assumptions about the preempt context having
a fixed id, reserved from use by normal user contexts, we may only
allocate the i915_gem_context when required. Then the subsequent
decisions on using preemption reduce to having the preempt context
available.

v2: Include an assert that we don't allocate the preempt context twice.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Acked-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207210544.26351-3-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>

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