dri3: Make dri3_get_supported_modifiers report no modifiers when using secondary GPU outputs

Since commit 9b899941 ("glamor: Enable modifier support for xfree86 too")
dri3_get_supported_modifiers will report the modifiers available on
buffer-objects on the primary GPU.

When using an output on a secondary GPU in combination with present flipping
this leads to Xorg.log filling up with these errors:

[ 28843.414] (WW) modeset(0): Page flip failed: Invalid argument
[ 28843.414] (EE) modeset(0): present flip failed
[ 28843.595] (WW) modeset(0): Page flip failed: Invalid argument
[ 28843.595] (EE) modeset(0): present flip failed

This is caused by drmmode_bo_import() failing on the secondary GPU because
the bo has modifiers which the secondary GPU does not support.

This commit fixes this by making dri3_get_supported_modifiers report
modifiers not being supported (as was done before commit 9b899941)
when a secondary GPU with outputs is attached through e.g.
xrandr --setprovideroutputsource

Related: #828

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
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