drm/amd/display: fix rotation_angle to use enum values

[Why]
Hardcoded fixed values are not proper.

[How]
Use enum values instead of fixed numbers.

Signed-off-by: Brandon Syu <Brandon.Syu@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>