drm/amd/display: Tidy up dce120_clock_source_create()

Also change sizeof to be automatic based on type declaration.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>