drm/i915: prevent crash with .disable_display parameter

drm/i915: prevent crash with .disable_display parameter

The .disable_display parameter was causing a fatal crash when fbdev
was dereferenced during driver init.

V1: protection in i915_drv.c
V2: Moved protection to intel_fbdev.c

Fixes: 43cee314345a ("drm/i915/fbdev: Limit the global async-domain synchronization")
Testcase: igt/drv_module_reload/basic-no-display
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Clint Taylor <clinton.a.taylor@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484775523-29428-1-git-send-email-clinton.a.taylor@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>