drm/i915: stop registering if drm_dev_register() fails

If drm_dev_register() fails there is no reason to continue registering
the driver and initializing.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210213042756.953007-1-lucas.demarchi@intel.com