drm/amd/display: Fix incorrect backlight register offset for DCN

[Why]
Typo in backlight refactor introduced wrong register offset.

[How]
SR(BIOS_SCRATCH_2) to NBIO_SR(BIOS_SCRATCH_2).
Signed-off-by: David Galiffi <David.Galiffi@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: <stable@vger.kernel.org>

drivers/gpu/drm/amd/display/dc/dce/dce_panel_cntl.h

@@ -54,7 +54,7 @@
 	SR(BL_PWM_CNTL2), \
 	SR(BL_PWM_PERIOD_CNTL), \
 	SR(BL_PWM_GRP1_REG_LOCK), \
-	SR(BIOS_SCRATCH_2)
+	NBIO_SR(BIOS_SCRATCH_2)
 
 #define DCE_PANEL_CNTL_SF(reg_name, field_name, post_fix)\
 	.field_name = reg_name ## __ ## field_name ## post_fix