drm/i915/gmbus: convert to struct intel_display

struct intel_display will replace struct drm_i915_private as the main
device pointer for display code. Switch gmbus code over to it.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d784e4799ab5095baa5c8fd840920066878c6273.1730146000.git.jani.nikula@intel.com