drm: Don't assign fbs for universal cursor support to files

The internal framebuffers we create to remap legacy cursor ioctls to
plane operations for the universal plane support shouldn't be linke to
the file like normal userspace framebuffers. This bug goes back to the
original universal cursor plane support introduced in

commit 161d0dc1
Author: Matt Roper <matthew.d.roper@intel.com>
Date:   Tue Jun 10 08:28:10 2014 -0700

    drm: Support legacy cursor ioctls via universal planes when possible (v4)

The isn't too disastrous since fbs are small, we only create one when the
cursor bo gets changed and ultimately they'll be reaped when the window
server restarts.

Conceptually we'd want to just pass NULL for file_priv when creating it,
but the driver needs the file to lookup the underlying buffer object for
cursor id. Instead let's move the file_priv linking out of
add_framebuffer_internal() into the addfb ioctl implementation, which is
the only place it is needed. And also rename the function for a more
accurate since it only creates the fb, but doesn't add it anywhere.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> (fix & commit msg)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> (provider of lipstick)
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>