drm/amd/display: fix mpcc assert condition

[Why]
In DCN2x asic, the MPCC status register definition changed, and
our logic for assert is incorrect. disabled is valid state,
where we should see idle and not busy, where as in not
disabled state, we should see not idle.

[How]
Change assert condition to be more sensible.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>