drm/i915/dp: convert g4x_dp.[ch] to struct intel_display

Going forward, struct intel_display shall replace struct
drm_i915_private as the main display device data pointer type. Convert
g4x_dp.[ch] to struct intel_display.

Some stragglers are left behind where needed.
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/b80ffb6373e9e3daaba0762ff7aebe168511b3a7.1725012870.git.jani.nikula@intel.com