drm/i915: Fix oops caused by fbdev initialization failure

drm/i915: Fix oops caused by fbdev initialization failure

intelfb_create() is called once on driver initialization. If it fails,
ifbdev->helper.fbdev, ifbdev->fb or ifbdev->fb->obj may be NULL.

Further up in the call stack, intel_fbdev_initial_config() calls
intel_fbdev_fini() to tear down the ifbdev on failure. This calls
intel_fbdev_destroy() which dereferences ifbdev->fb. Fix the ensuing
oops.

Also check in these functions if ifbdev is not NULL to avoid oops:

i915_gem_framebuffer_info() is called on access to debugfs file
"i915_gem_framebuffer" and dereferences ifbdev, ifbdev->helper.fb
and ifbdev->helper.fb->obj.

intel_connector_add_to_fbdev() / intel_connector_remove_from_fbdev()
are called when registering / unregistering an mst connector and
dereference ifbdev.

v3: Drop additional null pointer checks in intel_fbdev_set_suspend(),
    intel_fbdev_output_poll_changed() and intel_fbdev_restore_mode()
    since they already check if ifbdev is not NULL, which is sufficient
    now that intel_fbdev_fini() is called on initialization failure.
    (Requested by Daniel Vetter <daniel.vetter@ffwll.ch>)

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: http://patchwork.freedesktop.org/patch/msgid/d05f0edf121264a9d0adb8ca713fd8cc4ae068bf.1447938059.git.lukas@wunner.de
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>