drm/dp: Add macro to check max_downspread capability

Add a macro to check for the max_downspread capability in
drm_dp_helper.

Signed-off-by: Sankeerth Billakanti <quic_sbillaka@quicinc.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>

changes in v4:
    - Return 1 for DPCD version >= v1.1 (Stephen Boyd)
Link: https://lore.kernel.org/r/1635839325-401-4-git-send-email-quic_sbillaka@quicinc.com



Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>