drm/i915: Tidy error handling in i915_gem_init_hw

Let's reorder things so that we can do onion teardown rather than double
goto.

References: b96f6ebf ("drm/i915: Correctly handle error path in i915_gem_init_hw")
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@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/20180712124810.25241-1-michal.winiarski@intel.com