mesa/main: clean up ES2_compatibility check

This makes the logic a little bit easier to follow; this is *either*
about ES2 compatibility *or* about gles. GL_RGB565 was added already in
OpenGL ES 1.0.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/main/glformats.c

@@ -2325,7 +2325,7 @@ _mesa_base_tex_format(const struct gl_context *ctx, GLint internalFormat)
       }
    }
 
-   if (ctx->Extensions.ARB_ES2_compatibility) {
+   if (_mesa_has_ARB_ES2_compatibility(ctx) || _mesa_is_gles(ctx)) {
       switch (internalFormat) {
       case GL_RGB565:
          return GL_RGB;