drm/i915: move gmbus setup down to intel_modeset_init()

Pair the gmbus setup and teardown in the same layer. This also fixes the
double gmbus teardown on the i915_driver_modeset_probe() error path.

Move the gmbus setup a bit later in the sequence to make the follow-up
refactoring easier, and to pinpoint any unexpected consequences of this
change right here, instead of the later refactoring.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004122019.12009-3-jani.nikula@intel.com