drm/amd/display: Restore should_update_pstate_support after bad revert

[Why]
This function was mistakenly reverted as part of a legitimate revert. The
old version that was reverted to has bad logic, and is causing situations
where p-state change support is being toggled when it shouldn't be,
resulting in hangs.

Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>