drm/i915/display: avoid warnings when registering dual panel backlight

drm/i915/display: avoid warnings when registering dual panel backlight

Commit 20f85ef89d94 ("drm/i915/backlight: use unique backlight device
names") added support for multiple backlight devices on dual panel
systems, but did so with error handling on -EEXIST from
backlight_device_register(). Unfortunately, that triggered a warning in
dmesg all the way down from sysfs_add_file_mode_ns() and
sysfs_warn_dup().

Instead of optimistically always attempting to register with the default
name ("intel_backlight", which we have to retain for backward
compatibility), check if a backlight device with the name exists first,
and, if so, use the card and connector based name.

v2: reworked on top of the patch commit 20f85ef89d94
("drm/i915/backlight: use unique backlight device names")
v3: fixed the ref count leak(Jani N)

Fixes: 20f85ef89d94 ("drm/i915/backlight: use unique backlight device names")
Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220808035750.3111046-1-arun.r.murthy@intel.com