drm/edid/firmware: stop using a throwaway platform device

We've used a temporary platform device for firmware EDID loading since
it was introduced in commit da0df92 ("drm: allow loading an EDID as
firmware to override broken monitor"), but there's no explanation why.

Using a temporary device does not play well with CONFIG_FW_CACHE=y,
which caches firmware images (e.g. on suspend) so that drivers can
request firmware when the system is not ready for it, and return the
images from the cache (e.g. during resume). This works automatically for
regular devices, but obviously not for a temporarily created device.

Stop using the throwaway platform device, and use the drm device
instead.

Note that this may still be problematic for cases where the display was
plugged in during suspend, and the firmware wasn't loaded and therefore
not cached before suspend.

References: https://lore.kernel.org/r/[email protected]
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/2061
Reported-by: Matthieu CHARETTE <[email protected]>
Tested-by: Matthieu CHARETTE <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Acked-by: Thomas Zimmermann <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: jnikula

drivers/gpu/drm/drm_edid_load.c

@@ -172,20 +172,9 @@ static const struct drm_edid *edid_load(struct drm_connector *connector, const c
 		fwdata = generic_edid[builtin];
 		fwsize = sizeof(generic_edid[builtin]);
 	} else {
-		struct platform_device *pdev;
 		int err;
 
-		pdev = platform_device_register_simple(connector->name, -1, NULL, 0);
-		if (IS_ERR(pdev)) {
-			drm_err(connector->dev,
-				"[CONNECTOR:%d:%s] Failed to register EDID firmware platform device for connector \"%s\"\n",
-				connector->base.id, connector->name,
-				connector->name);
-			return ERR_CAST(pdev);
-		}
-
-		err = request_firmware(&fw, name, &pdev->dev);
-		platform_device_unregister(pdev);
+		err = request_firmware(&fw, name, connector->dev->dev);
 		if (err) {
 			drm_err(connector->dev,
 				"[CONNECTOR:%d:%s] Requesting EDID firmware \"%s\" failed (err=%d)\n",