drm: fix error returns, sysfs error handling

- callers of drm_sysfs_create() and drm_sysfs_device_add() looked for
  errors using IS_ERR(), but the functions themselves only ever returned
  NULL on error.  Fixed.

- unwind from, and propagate sysfs errors

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>