drm/i915/gmbus: mass dev_priv -> i915 rename

Now that gmbus no longer uses macros that assume dev_priv is implicitly
available, mass rename dev_priv to i915 in gmbus code.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fcf16a65f7975379a887ed57c623b25de7b344c8.1661855191.git.jani.nikula@intel.com