drm/amd/display: Clear OPTC mem select on disable

[Why]
Not clearing the memory select bits prior to OPTC disable can cause DSC
corruption issues when attempting to reuse a memory instance for another
OPTC that enables ODM.

[How]
Clear the memory select bits prior to disabling an OPTC.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ilya Bakoulin <ilya.bakoulin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>