drm/i915: add i915_driver_modeset_remove()

For completeness, add counterpart to i915_driver_modeset_probe() and
remove the asymmetry in the probe/remove parts. No functional changes.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920185421.17822-1-jani.nikula@intel.com