drm/amd/display: fix wrong register access

[why]
fw version check was for release branch.
for staging, it has a chance to enter wrong code path.

Reviewed-by: Hansen Dsouza <hansen.dsouza@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>