drm/amd/display: Separate set_gsl from set_gsl_source_select

[Why/How]
Separate the checks for set_gsl and set_gsl_source_select, since
source_select may not be implemented/necessary.

Reviewed-by: Nevenko Stupar <nevenko.stupar@amd.com>
Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>