video: fsl_dcu_fb: Fix DCU_MODE_BLEND_ITER setting

DCU_LAYER_MAX_NUM is currently used for DCU_MODE_BLEND_ITER and it
actually overflows the maximum value of BLEND_ITER for Vybrid and
LS102XA. Fix this by using a default value of 2.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>