drm/i915: Sparse warning about invalid value for burst_ena in tv_modes

drivers/gpu/drm/i915/intel_tv.c|479 col 16| warning: cast truncates bits
from constant value (8 becomes 0)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>