drm/i915/ddi: drop unused but set variable intel_dp

Prepare for re-enabling -Wunused-but-set-variable.

The intel_dp variable has been unused since commit ef79fafe
("drm/i915: Eliminate intel_dp.regs.dp_tp_{ctl,status}").

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/66ca543b400a2048a6a84bb57a7bac8943014a96.1685119007.git.jani.nikula@intel.com