drm/i915/dmc: mass rename dev_priv to i915

Follow the contemporary convention for struct drm_i915_private * naming.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230301122944.1298929-5-jani.nikula@intel.com