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

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: 1454642960b0 ("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 3f797396d7f4eb9bb6eded184bbc6f033628a6f6)
Cc: stable@vger.kernel.org