drm/i915: simplify i915_gem_init_early

i915_gem_init_early doesn't need to return anything.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927173409.31175-3-matthew.auld@intel.com