drm/amd/display: Tidy up dce120_timing_generator_enable_advanced_request()

Simplify the function by removing identical looking code blocks.

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