drm/amd/display: Don't always set pstate true if dummy latency = 0

[Why]
If dummy pstate latency is 0 we should report the actual
pstate support, and not that pstate is always supported.

[How]
Don't always program pstate support true

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>