drm/radeon: initialize backlight for iMac12,2 with Radeon 6750M

drm/radeon: initialize backlight for iMac12,2 with Radeon 6750M

If a Radeon 6750M GPU from an iMac12,1 is installed into an
iMac 12,2, there is no backlight device initialized during
boot. Everything else is functional, but the display brightness
cannot be controlled. There are no directories present in
/sys/class/backlight after booting. A simple one line
modification to an if statement fixes this issue by initializing
the radeon backlight device for an iMac12,2 as well if it has a
6750M. After the patch, brightness can be controlled and
radeon_bl0 is present in /sys/class/backlight. This was tested
by compiling the latest kernel with and without the patch.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3378
Signed-off-by: Kendall Smith <kendallsm2@icloud.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>