glamor: Unbreak glamor_fd_from_pixmap()

When support for allocating GBM BOs with modifiers was added,
glamor_fd_from_pixmap() was changed so that it would return an error if
it got a bo with modifiers set from glamor_fds_from_pixmap(). The
problem is that on systems that support BOs with modifiers,
glamor_fds_from_pixmap() will always return BOs with modifiers.

This means that glamor_fd_from_pixmap() was broken entirely, which broke
a number of other things including glamor_shareable_fd_from_pixmap(),
which meant that modesetting using multiple GPUs with the modesetting
DDX was also broken. Easy reproducer:

- Find a laptop with DRI prime that has outputs connected to the
  dedicated GPU and integrated GPU
- Try to enable one display on each using the modesetting DDX
- Fail

Since there isn't a way to ask for no modifiers from
glamor_fds_from_pixmap, we create a shared _glamor_fds_from_pixmap()
function used by both glamor_fds_from_pixmap() and
glamor_fd_from_pixmap() that calls down to the appropriate
glamor_egl_fd*_from_pixmap() function.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Cc: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Fixes: c8c276c956 ("glamor: Implement PixmapFromBuffers and BuffersFromPixmap")
(cherry picked from commit 186a21c4ba)

hw/xwayland/xwayland-glamor-gbm.c

@@ -517,6 +517,16 @@ glamor_egl_fds_from_pixmap(ScreenPtr screen, PixmapPtr pixmap, int *fds,
 #endif
 }
 
+/* Not actually used, just defined here so there's something for
+ * _glamor_egl_fds_from_pixmap() to link against
+ */
+_X_EXPORT int
+glamor_egl_fd_from_pixmap(ScreenPtr screen, PixmapPtr pixmap,
+                          CARD16 *stride, CARD32 *size)
+{
+    return -1;
+}
+
 _X_EXPORT Bool
 glamor_get_formats(ScreenPtr screen,
                    CARD32 *num_formats, CARD32 **formats)