kmsgrab: Use invalid modifier if modifiers weren't used.

kmsgrab: Use invalid modifier if modifiers weren't used.

The kernel defaults to initializing the field to 0 when modifiers
are not used and this happens to be linear. If we end up actually
passing the modifier to a driver, tiling issues happen.

So if the kernel doesn't return a modifier set it explicitly to
INVALID. That way later processing knows there is no explicit
modifier.

Signed-off-by: Mark Thompson <sw@jkqxz.net>