drm/i915: Clean up DP code local variables and calling conventions

Eliminate a ton of pointless 'dev' variables in the DP code, and pass
around 'dev_priv' instead of 'dev'.

v2: Rebase
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171109152758.32257-1-ville.syrjala@linux.intel.com