drm/i915: Move pinning of dev_priv->kernel_context into its creator

Rather than have every context ask "am I owned by the kernel? pin!",
move that logic into the creator of the kernel context, in order to
improve code comprehension.

v2: Throw away the user_handle on failure to allocate the ppgtt.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464098023-3294-5-git-send-email-chris@chris-wilson.co.uk