drm/i915: remove an extra #define for DP_RECEIVER_CAP_SIZE

Identical #define is now available in include/drm/drm_dp_helper.h, nuke the
dupe.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>