drm/i915: Fix oops in overlay due to frontbuffer tracking

drm/i915: Fix oops in overlay due to frontbuffer tracking

The vma will be NULL if the overlay was previously off, so
dereferencing it will oops. Check for NULL before doing that.

Cc: stable@vger.kernel.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Fixes: b7bca067f86d ("drm/i915/overlay: Use VMA as the primary tracker for images")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481131693-27993-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit 4a15cdbbc55463e55a7cdcf33f84ccc742ca9c29)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>