drivers, drm: fix qxl build error when debugfs is disabled

Fix build error when CONFIG_DEBUG_FS is disabled:

drivers/gpu/drm/qxl/qxl_debugfs.c: In function 'qxl_debugfs_init':
drivers/gpu/drm/qxl/qxl_debugfs.c:76:2: error: implicit declaration of function 'drm_debugfs_create_files'
drivers/gpu/drm/qxl/qxl_debugfs.c: In function 'qxl_debugfs_takedown':
drivers/gpu/drm/qxl/qxl_debugfs.c:84:2: error: implicit declaration of function 'drm_debugfs_remove_files'

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>