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/qt/qt.c

@@ -38,7 +38,7 @@ qt_nvr_save(void)
 char icon_set[256] = ""; /* name of the iconset to be used */
 
 int
-plat_vidapi(char *api)
+plat_vidapi(const char *api)
 {
     if (!strcasecmp(api, "default") || !strcasecmp(api, "system")) {
         return 0;