drm/i915/dp: hide dp_to_i915() inside intel_dp.c

Now that only intel_dp.c uses dp_to_i915(), hide it there. This removes
a header dependency on to_i915().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e214aa6991aea4fc878b36dcd3eaece9f1fba592.1725012870.git.jani.nikula@intel.com