xf86drmMode: set FB_MODIFIERS flag when modifiers are supplied

The kernel will always return EINVAL if modifiers are supplied but
the flag DRM_MODE_FB_MODIFIERS isn't set. That's a pretty nice
footgun.

Be a little more helpful and set the flag if the user has supplied
a modifier array.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>