drm/i915: introduce dp_to_i915() helper

No functional change. But let's get first i915 pointer
directly from intel_dp so we can clean up a lot of code
later.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180827223021.7145-1-rodrigo.vivi@intel.com