drm/i915: Remove num_pages parameter to i915_error_object_create()

For cleanliness, i915_error_object_create() was written to handle the
NULL pointer in a central location. The macro that wrapped it and passed
it a num_pages to use, was not safe. As we now never limit the num_pages
to use (we did so at one point to only capture the first page of the
context), we can remove the redundant macro and be NULL safe again.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>