drm/amd/display: Only read ACPI backlight caps once

[WHY]
Backlight caps are read already in amdgpu_dm_update_backlight_caps().
They may be updated by update_connector_ext_caps(). Reading again when
registering backlight device may cause wrong values to be used.

[HOW]
Use backlight caps already registered to the dm.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 148144f6d2f14b02eaaa39b86bbe023cbff350bd)
Cc: stable@vger.kernel.org