drm: xilinx: drv: Unset set_busid callback

drm: xilinx: drv: Unset set_busid callback

The DRM framework sets the busid by itself when the set_busid
callback is not set by the driver. Applications can use
the busid to differentiate when there are multiple instances,
thus don't register the set_busid callback.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Tested-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>