drm/prime: Honor requested file flags when exporting a buffer

The DRM PRIME API passes file flags to the driver for the exported
buffer. Honor them instead of hardcoding 0600.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>