mesa: remove rgbMode check in enable_texture()

If the currently bound FBO isn't yet validated it's possible for
rgbMode to be zero so we'll lose the texture enable.
This could fix some FBO rendering glitches, but I don't know of
any specific instances.

src/mesa/main/enable.c

@@ -231,7 +231,7 @@ enable_texture(GLcontext *ctx, GLboolean state, GLbitfield bit)
    const GLuint newenabled = (!state)
        ? (texUnit->Enabled & ~bit) :  (texUnit->Enabled | bit);
 
-   if (!ctx->DrawBuffer->Visual.rgbMode || texUnit->Enabled == newenabled)
+   if (texUnit->Enabled == newenabled)
        return GL_FALSE;
 
    FLUSH_VERTICES(ctx, _NEW_TEXTURE);