drm/amd/display: Correct updating logic of dcn21's pipe VM flags

[Why]:
Renoir's pipe VM flags are not correctly updated if pipe strategy has
changed during some scenarios. It will result in watermarks mistakenly
calculation, thus underflow and garbage appear.

[How]:
Correctly update pipe VM flags to pipes which have been populated.

Signed-off-by: Dale Zhao <dale.zhao@amd.com>
Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>