drm/i915/gmbus: alloc intel_gmbus dynamically

Allocate the individual intel_gmbus structs dynamically. This lets us
hide struct intel_gmbus inside intel_gmbus.c completely. Also use the
cleanup function on the error path to avoid duplication.

Leave #include <linux/i2c.h> in i915_drv.h for now, as it pulls in a
bunch of implicit dependencies.

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/20220303181931.1661767-4-jani.nikula@intel.com