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>

include/drm/drm_dp_helper.h

@@ -73,6 +73,7 @@
 # define DP_ENHANCED_FRAME_CAP		    (1 << 7)
 
 #define DP_MAX_DOWNSPREAD                   0x003
+# define DP_MAX_DOWNSPREAD_0_5		    (1 << 0)
 # define DP_NO_AUX_HANDSHAKE_LINK_TRAINING  (1 << 6)
 
 #define DP_NORP                             0x004