drm/i915: pass i915 to i915_driver_modeset_probe()

In general, prefer struct drm_i915_private * over struct drm_device *
when either will do. Rename the local variable to i915. 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/20190920185421.17822-2-jani.nikula@intel.com