drm/i915: Nuke local copies/pointers of intel_dp->DP

Get rid of the local copies and pointers of intel_dp->DP and
instead just poke at it directly. Makes it much easier to see
where it actually gets used/modified.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930134310.31669-4-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>