dri3: Track full 64-bit SBC numbers, instead of just 32-bits

dri3: Track full 64-bit SBC numbers, instead of just 32-bits

Tracking the full 64-bit SBC values makes it clearer how those values are
being used, and simplifies the wait_msc code. The only trick is in
re-constructing the full 64-bit value from Present's 32-bit serial number
that we use to pass the SBC value from request to event.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Eric Anholt <eric@anholt.net>