Upon review, all path share the same dependencies for updating the
registers and so we can benefit from sharing the code and checking
early.

This removes the unsightly intel_wait_for_vblank() from the lowlevel
functions and upon further analysis the only path that will require a
wait is if we are performing an instantaneous transition between two
valid FBC configurations. The page-flip path itself will have disabled
FBC registers and will have waited for at least one vblank before
finishing the flip and attempting to re-enable FBC. This wait can be
accomplished simply by delaying the enable until after we are sure that
a vblank will have passed, which we are already doing to make sure that
the display is settled before enabling FBC.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NKeith Packard <keithp@keithp.com>
Signed-off-by: NKeith Packard <keithp@keithp.com>