drm/i915/bios: throw away high level child device union

All the child device config fields, including legacy, are now available
in the same struct, so use it for everything.

As this change touches plenty of code with "p_child", rename them to
"child" while at it. Also do some simple unification and constification
where not intrusive. This in the name of avoiding extra cleanup churn
for the same lines as here.

No functional changes.

Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/103300a9ae8629624619fc8df2c533e745cc5a78.1503600621.git.jani.nikula@intel.com