drm: Provide platform module-init macro

Provide a helper macro to register platform DRM drivers. The new
macro behaves like module_platform_driver() with an additional
test if DRM modesetting has been enabled.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211222082831.196562-7-javierm@redhat.com