drm/amd/display: Avoid operating on copies of backlight caps

Making a copy of the backlight caps structure between uses is unnecessary.
Refer to pointers to the same structure when using it.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Link: https://lore.kernel.org/r/20250228185145.186319-4-mario.limonciello@amd.com


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>