drm: move DP_MAX_DOWNSTREAM_PORTS from i915 to drm core

DP_MAX_DOWNSTREAM_PORTS=0x10 is a vendor-independent constant.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190829114854.1539-1-oleg.vasilev@intel.com