drm/amd/display: check if ext_caps is valid in BL setup

LVDS connectors don't have extended backlight caps so check
if the pointer is valid before accessing it.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/5012
Fixes: 1454642 ("drm/amd: Re-introduce property to control adaptive backlight modulation")
Cc: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 3f79739)
Cc: stable@vger.kernel.org

Author: alexdeucher

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

@@ -5423,7 +5423,7 @@ static void setup_backlight_device(struct amdgpu_display_manager *dm,
 	caps = &dm->backlight_caps[aconnector->bl_idx];
 
 	/* Only offer ABM property when non-OLED and user didn't turn off by module parameter */
-	if (!caps->ext_caps->bits.oled && amdgpu_dm_abm_level < 0)
+	if (caps->ext_caps && !caps->ext_caps->bits.oled && amdgpu_dm_abm_level < 0)
 		drm_object_attach_property(&aconnector->base.base,
 					   dm->adev->mode_info.abm_level_property,
 					   ABM_SYSFS_CONTROL);