gallium: enable EGL_EXT_image_dma_buf_import_modifiers unconditionally

gallium: enable EGL_EXT_image_dma_buf_import_modifiers unconditionally

This is a re-do of [1].

Enable EGL_EXT_image_dma_buf_import_modifiers with
EXT_image_dma_buf_import. This allows users to use queryDmaBufFormats to
query the list of supported formats even if modifiers are not supported.

With this change, queryDmaBufModifiers always returns zero modifiers. A
compositor survey reveals that this should be fine: wlroots [2],
Weston [3], Mutter [4] [5], kwin [6] and xorg-xserver [7] seem to all
support this case gracefully.

Tested with Sway and wlroots by running weston-info and checking the
list of formats advertised by zwp_linux_dmabuf_v1. Also ran weston-simple-egl
and checked zwp_linux_dmabuf_v1 was used instead of wl_drm.

[1]: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1812
[2]: https://github.com/swaywm/wlroots/blob/8707a9b7ecbba0321804604d9ea954a46ecced21/render/egl.c#L629
[3]: https://gitlab.freedesktop.org/wayland/weston/-/blob/786490cb53439624fd3c20b9e19d3ea5ec316c00/libweston/renderer-gl/gl-renderer.c#L2337
[4]: https://gitlab.gnome.org/GNOME/mutter/-/blob/f0df07cba3ca308b47c9aefcc8112e8880fd9950/src/wayland/meta-wayland-dma-buf.c#L486
[5]: https://gitlab.gnome.org/GNOME/mutter/-/blob/0a6034ef3a745c25ab63c2ca8d4ae08bc5e09d88/src/backends/native/meta-renderer-native.c#L399
[6]: https://cgit.kde.org/kwin.git/tree/platformsupport/scenes/opengl/egl_dmabuf.cpp?id=9b7ab4d16a8ee0cb35108362ee5aa046f4ae20b7#n473
[7]: https://gitlab.freedesktop.org/xorg/xserver/-/blob/26004df63c25061586a967f3586795a75280acc2/glamor/glamor_egl.c#L682

Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4298>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4298>