drm/i915/gmch: mass rename dev_priv to i915

Prefer the contemporary naming.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3e4aaadeb4a027165f5724027963aa5e8d747190.1673958757.git.jani.nikula@intel.com