drm/i915/backlight: Use drm helper to initialize edp backlight

Now that drm_edp_backlight init has been modified to take
into account the setup of lumininace based brightness manipulation
we can just use that.

--v2
-Fix commit message [Arun]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://lore.kernel.org/r/20250620063445.3603086-12-suraj.kandpal@intel.com