drm/i915/backlight: use unique backlight device names

Registering multiple backlight devices with intel_backlight name will
obviously fail, regardless of whether they're two connectors in the same
drm device or two different drm devices.

It would be preferrable to switch to completely unique names, and sunset
the generic intel_backlight name. However, there are apparently users
out there that hardcode the name, so the change would break backward
compatibility.

As a compromise, register the first device with intel_backlight name. In
the common case, this is the only backlight device anyway. From the
second device on, use card%d-%s-backlight format, for example
card0-eDP-2-backlight, to make the name unique.

This approach does not preclude us from registering the first device
using the same naming scheme in the future.

v2: Keep using intel_backlight name for first backlight device

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2794


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/7dc3f6974711ce44522189dc9db05d1e6e24e6d8.1619604743.git.jani.nikula@intel.com