drm/radeon: Set MASTER_UPDATE_MODE to 0 again

With the previous change, it's safe to let page flips take effect
anytime during a vertical blank period.

This can avoid delaying a flip by a frame in some cases where we get to
radeon_flip_work_func -> adev->mode_info.funcs->page_flip during a
vertical blank period.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>