drm/i915/backlight: clean up backlight device register

Add connector and backlight device name to logging, and propagate error
code from backlight_device_register() instead of flattening to
-ENODEV. Storing the name in an allocated buffer is unnecessary here,
but makes follow-up work on names much cleaner.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/271206461d9c0f42755792236330b588df3b532e.1619604743.git.jani.nikula@intel.com