drm/amd/display: remove macro which is in header already

[Why & How]
Remove temp macro since the enum is in header file already.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>