drm/amd/display: Fix compile error with ACPI disabled

The fallback code for getting default backlight caps was using
the wrong variable name.  Fix it.

Fixes: https://lists.freedesktop.org/archives/dri-devel/2018-November/197752.htmlSigned-off-by: David Francis <David.Francis@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -1700,8 +1700,8 @@ static void amdgpu_dm_update_backlight_caps(struct amdgpu_display_manager *dm)
 				AMDGPU_DM_DEFAULT_MAX_BACKLIGHT;
 	}
 #else
-	dm->backlight_min_input_signal = AMDGPU_DM_DEFAULT_MIN_BACKLIGHT;
-	dm->backlight_max_input_signal = AMDGPU_DM_DEFAULT_MAX_BACKLIGHT;
+	dm->backlight_caps.min_input_signal = AMDGPU_DM_DEFAULT_MIN_BACKLIGHT;
+	dm->backlight_caps.max_input_signal = AMDGPU_DM_DEFAULT_MAX_BACKLIGHT;
 #endif
 }