drm/amd/display: Lock DC and exit IPS when changing backlight

Backlight updates require aux and/or register access. Therefore, driver
needs to disallow IPS beforehand.

So, acquire the dc lock before calling into dc to update backlight - we
should be doing this regardless of IPS. Then, while the lock is held,
disallow IPS before calling into dc, then allow IPS afterwards (if it
was previously allowed).

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 988fe286)
Cc: stable@vger.kernel.org # 6.10+