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

drivers/gpu/drm/i915/i915_drv.h

@@ -1723,7 +1723,7 @@ struct intel_vbt_data {
 	int crt_ddc_pin;
 
 	int child_dev_num;
-	union child_device_config *child_dev;
+	struct child_device_config *child_dev;
 
 	struct ddi_vbt_port_info ddi_port_info[I915_MAX_PORTS];
 	struct sdvo_device_mapping sdvo_mappings[2];

drivers/gpu/drm/i915/intel_vbt_defs.h

@@ -275,7 +275,7 @@ struct old_child_dev_config {
  * a baseline for some of the VBT documentation. When adding new fields, please
  * include the BDB version when the field was added, if it's above that.
  */
-struct common_child_dev_config {
+struct child_device_config {
 	u16 handle;
 	u16 device_type;
 
@@ -347,17 +347,6 @@ struct common_child_dev_config {
 	u8 iboost_level;
 } __packed;
 
-
-/* This field changes depending on the BDB version, so the most reliable way to
- * read it is by checking the BDB version and reading the raw pointer. */
-union child_device_config {
-	/* This one should only be kept for legacy code. */
-	struct old_child_dev_config old;
-	/* This one should also be safe to use anywhere, even without version
-	 * checks. */
-	struct common_child_dev_config common;
-} __packed;
-
 struct bdb_general_definitions {
 	/* DDC GPIO */
 	u8 crt_ddc_gmbus_pin;