drm/amd/display: Clear MASTER_UPDATE_LOCK_DB_EN when disable doublebuffer lock

[Why]
It seems there's a typo to set MASTER_UPDATE_LOCK_DB_EN when disable
doublebuffer lock.

[How]
Clear MASTER_UPDATE_LOCK_DB_EN when disable doublebuffer lock

Signed-off-by: Robin Chen <robin.chen@amd.com>
Reviewed-by: Joshua Aberback <Joshua.Aberback@amd.com>
Acked-by: Wayne Lin <waynelin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>