media: davinci_vpfe: vpfe_video: remove an unused var

as warned:

  drivers/staging/media/davinci_vpfe/vpfe_video.c: In function 'vpfe_streamon':
  drivers/staging/media/davinci_vpfe/vpfe_video.c:1471:31: warning: variable 'sdinfo' set but not used [-Wunused-but-set-variable]
    struct vpfe_ext_subdev_info *sdinfo;
                               ^~~~~~

While here, cleanup this kernel-doc warning:

  drivers/staging/media/davinci_vpfe/vpfe_video.c:225: warning: Function parameter or member 'pipe' not described in 'vpfe_video_validate_pipeline'

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>