drm/amd/display: fix an incorrect ODM policy assigned for subvp

drm/amd/display: fix an incorrect ODM policy assigned for subvp

[why]
When Subvp pipe's index is smaller than main pipe's index, the main
pipe's ODM policy is not yet assigned. If we assign subvp pipe's ODM
policy based on main pipe, we will assign uninitialized ODM policy.

[how]
Instead of copying main pipe's policy we copy the main pipe ODM policy
logic. So it doesn't matter whether if main pipe's ODM policy is set,
phantom pipe will always have the same policy because it running the
same calcualtion to derive ODM policy.

Cc: stable@vger.kernel.org
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>