gallium: enable EGL_EXT_image_dma_buf_import_modifiers unconditionally

Simon Ser requested to merge emersion/mesa:export-dmabuf-modifiers into master

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.

cc @ajax @daenzer @daniels

Edited by Simon Ser

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