drm/amd/display: Assign correct left shift

[Why]
Reading for DP alt registers return incorrect values due to LE_SF
definition missing.

[How]
Define correct LE_SF or DP alt registers.

Signed-off-by: Chris Park <Chris.Park@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>