drm/i2c/tda998x: Remove unused save/restore drm encoder helpers.

save/restore have been removed from drm_encoder_helper_funcs by
'commit 79f13ad5d8e0 ("drm: Move encoder->save/restore into nouveau")'

But this module was still defining it with empty content causing
compilation fails:

drivers/gpu/drm/i2c/tda998x_drv.c:1354:10: warning: initialization from
incompatible pointer type [-Wincompatible-pointer-types]
  .save = tda998x_encoder_save,

drivers/gpu/drm/i2c/tda998x_drv.c:1355:2: error: unknown field 'restore'
specified in initializer
  .restore = tda998x_encoder_restore,

Cc: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449513306-17309-1-git-send-email-rodrigo.vivi@intel.com