drm/gma500: Remove DP_LINK_STATUS_SIZE redefinition

The DRM/DP helpers already contain a definition for this macro. Remove
the duplicate in the GMA500 driver to avoid having to keep both updated
synchronously.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>