drm/i915: name intel dp hooks per platform

In line with the rest of the code base. No functional changes.

v2: also s/intel_pre_enable_dp/g4x_pre_enable_dp/ for consistency (Ville)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>