egl: warn user if they set an invalid EGL_PLATFORM

Technically, the user might have set EGL_DISPLAY instead of
EGL_PLATFORM, but since the former is deprecated let's just mention the
latter in the warning message.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>

src/egl/main/egldisplay.c

@@ -105,6 +105,9 @@ _eglGetNativePlatformFromEnv(void)
       }
    }
 
+   if (plat == _EGL_INVALID_PLATFORM)
+      _eglLog(_EGL_WARNING, "invalid EGL_PLATFORM given");
+
    return plat;
 }