drm/amd/display: Set vsc pack revision when DPCD revision is >= 1.2

Brightness couldn't change when booting up in DC mode.
It was because "psr_enabled" flag was not set to true before
setting vsc packet revision, causing packet rev setup was skipped.
Now instead of checking the psr flag, it checks if the DPCD_REV >= 1.2
and set the vsc packet revision.

Signed-off-by: Tao <xtao@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>