tests/kms_flip: Move EBUSY to a new busy-flip subtest

tests/kms_flip: Move EBUSY to a new busy-flip subtest

The EBUSY checking is very fragile currently in case there's any kind
extra delay in the test loop. At least the flip-vs-rmfb fails reliably
on my IVB.

So to make the test less fragile remove the EBUSY check from all the
current flip tests, and instead add a specific busy-flip test. To
better guarantee that we get the EBUSY add a blocking vblank wait just
before we issue the first flip. This maximizes the time we have to
submit two flips during one frame.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>