drm/i915/gem: Tidy up error handling for eb_parse()

As the caller no longer uses the i915_vma result, stop returning it and
just return the error code instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191211110437.4082687-4-chris@chris-wilson.co.uk