cl-tone-map: fix y_target to prevent large gain
cl-tone-map: fix y_target to prevent large gain

 * modification for y_target to avoid too large gain when image
   environment too dark

Signed-off-by: Wind Yuan <d3b25194a13defcddf619d66a03bde7d96f8d487@intel.com>

dspmeng/libxcam,dspmeng/libxcam,dspmeng/libxcam