[media] s5p_mfc: get rid of several warnings

drivers/media/platform/s5p-mfc/s5p_mfc_dec.c:826:5: warning: no previous prototype for 'vidioc_decoder_cmd' [-Wmissing-prototypes]
 int vidioc_decoder_cmd(struct file *file, void *priv,
     ^
drivers/media/platform/s5p-mfc/s5p_mfc.c: In function 's5p_mfc_runtime_resume':
drivers/media/platform/s5p-mfc/s5p_mfc.c:1314:6: warning: variable 'pre_power' set but not used [-Wunused-but-set-variable]
  int pre_power;
      ^
drivers/media/platform/s5p-mfc/s5p_mfc_cmd_v5.c:163:25: warning: no previous prototype for 's5p_mfc_init_hw_cmds_v5' [-Wmissing-prototypes]
 struct s5p_mfc_hw_cmds *s5p_mfc_init_hw_cmds_v5(void)
                         ^
drivers/media/platform/s5p-mfc/s5p_mfc_cmd_v6.c:156:25: warning: no previous prototype for 's5p_mfc_init_hw_cmds_v6' [-Wmissing-prototypes]
 struct s5p_mfc_hw_cmds *s5p_mfc_init_hw_cmds_v6(void)
                         ^
drivers/media/platform/s5p-mfc/s5p_mfc_opr_v5.c: In function 's5p_mfc_run_dec_frame':
drivers/media/platform/s5p-mfc/s5p_mfc_opr_v5.c:1189:15: warning: variable 'index' set but not used [-Wunused-but-set-variable]
  unsigned int index;
               ^
drivers/media/platform/s5p-mfc/s5p_mfc_enc.c: In function 'cleanup_ref_queue':
drivers/media/platform/s5p-mfc/s5p_mfc_enc.c:742:27: warning: variable 'mb_c_addr' set but not used [-Wunused-but-set-variable]
  unsigned long mb_y_addr, mb_c_addr;
                           ^
drivers/media/platform/s5p-mfc/s5p_mfc_enc.c:742:16: warning: variable 'mb_y_addr' set but not used [-Wunused-but-set-variable]
  unsigned long mb_y_addr, mb_c_addr;
                ^
drivers/media/platform/s5p-mfc/s5p_mfc_enc.c: At top level:
drivers/media/platform/s5p-mfc/s5p_mfc_enc.c:1684:5: warning: no previous prototype for 'vidioc_encoder_cmd' [-Wmissing-prototypes]
 int vidioc_encoder_cmd(struct file *file, void *priv,
     ^

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>