drm/amd/display: Only program P-State force if pipe config changed

[Description]
Today for MED update type we do not call update clocks. However, for FPO
the assumption is that update clocks should be called to disable P-State
switch before any HW programming since FPO in FW and driver are not
synchronized. This causes an issue where on a MED update, an FPO P-State
switch could be taking place, then driver forces P-State disallow in the below
code and prevents FPO from completing the sequence. In this case we add a check
to avoid re-programming (and thus re-setting) the P-State force register by
only reprogramming if the pipe was not previously Subvp or FPO. The assumption
is that the P-State force register should be programmed correctly the first
time SubVP / FPO was enabled, so there's no need to update / reset it if the
pipe config has never exited SubVP / FPO.

Reviewed-by: Samson Tam <samson.tam@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>