drm/arm/hdlcd: Replace module initialization with DRM helpers

Replace module_platform_driver() with drm_module_platform_driver(). The
DRM macro respects drm_firmware_drivers_only() and fails if the flag has
been set.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211222082831.196562-10-javierm@redhat.com