drm/i915: simplify assignments inside intel_dp.c

 - Replace container_of with enc_to_intel_dp.
 - Walk through less structures when making assignments.
 - Rename some variables to keep our naming standards.

As a bonus, this will reduce the usage of "struct intel_dp", making
the future patch that introduces intel_digital_port smaller and easier
to review.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>