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: c8c276c9 ("glamor: Implement PixmapFromBuffers and BuffersFromPixmap")