checkasm/vf_blend: use the correct depth parameters to initialize the blend modes

This effectively enables the tests that until now were just running
the C version alone.

Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 55e1bc39cb)

tests/checkasm/vf_blend.c

@@ -99,7 +99,7 @@ void checkasm_check_blend(void)
 
 #define check_and_report(name, val, depth)        \
     param.mode = val;                             \
-    ff_blend_init(&param, depth - 1);             \
+    ff_blend_init(&param, depth * 8);             \
     if (check_func(param.blend, #name))           \
         check_blend_func(depth);