drm/dp: Add define to set 0.5% down-spread in MAX_DOWNSPREAD register

Support of 0.5% down-spread is required for DisplayPort Specification
Version 1.1 Sink.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
[treding@nvidia.com: rename to DP_MAX_DOWNSPREAD_0_5]
Signed-off-by: Thierry Reding <treding@nvidia.com>