st/dri: Initialise modifier to INVALID for DRI2

st/dri: Initialise modifier to INVALID for DRI2

When allocating a buffer for DRI2, set the modifier to INVALID to inform
the backend that we have no supplied modifiers and it should do its own
thing. The missed initialisation forced linear, even if the
implementation had made other decisions.

This resulted in VC4 DRI2 clients failing with:
  Modifier 0x0 vs. tiling (0x700000000000001) mismatch

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: Andreas Müller <schnitzeltony@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Fixes: 3f8513172ff6 ("gallium/winsys/drm: introduce modifier field to winsys_handle")