drm/amd/display: Modify logic for when to wait for mpcc idle

[Why]
I was advised that we may need to check for mpcc idle in more cases
than just when opp_changed is true. Also, mpcc_inst is equal to
pipe_idx, so remove for loop.

[How]
Remove opp_changed flag check and mpcc_inst loop.

Signed-off-by: Noah Abradjian <noah.abradjian@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>