Mark plat_vidapi() argument as const and remove the NULL

* plat_vidapi() accepts char pointer, which never gets modifed
later on. Mark it as const.

* In src/config.c, va_name is initialized as NULL, however,
plat_vidapi_name() never returns a NULL pointer, nor it was
initialized under a condition branch. Removing NULL, might
save one instruction, which requires zeroing the register
before setting its initial value.

src/config.c

@@ -1720,7 +1720,7 @@ save_general(void)
     char          temp[512];
     char          buffer[512] = { 0 };
 
-    const char *va_name = NULL;
+    const char *va_name;
 
     ini_section_set_int(cat, "vid_resize", vid_resize);
     if (vid_resize == 0)