drm/i915: use ERR_CAST instead of ERR_PTR/PTR_ERR

Inspired by scripts/coccinelle/api/err_cast.cocci

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>