drm/i915: free intel_fb

We need to free the allocated intel_fb in the error path, not
intel_fb->base. Otherwise we risk calling kfree with a non-kmalloc'd
address, which is bound to give us grief at some point.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471964444-24460-1-git-send-email-matthew.auld@intel.com