drm/i915/gmbus: use to_intel_gmbus() instead of open coding

We have a helper for getting at the enclosing gmbus struct from the
embedded i2c_adapter, use it.

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/20220304101426.1891347-2-jani.nikula@intel.com