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

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 da0df92b5731 ("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/20220727074152.43059-1-matthieu.charette@gmail.com
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/2061
Reported-by: Matthieu CHARETTE <matthieu.charette@gmail.com>
Tested-by: Matthieu CHARETTE <matthieu.charette@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114111709.434979-1-jani.nikula@intel.com