drm/i915: pass i915 to intel_modeset_driver_remove()

In general, prefer struct drm_i915_private * over struct drm_device *
when either will do. Rename the local variable to i915. Also propagate
to intel_hpd_poll_fini(). 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/20190920185421.17822-3-jani.nikula@intel.com