drm/i915/aux: rename dev_priv to i915

No reason to stick to dev_priv, rename to i915.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231011201533.1081368-2-jani.nikula@intel.com