drm/amd/display: Clean up dp_blank functions

[Why]
Unused variable "refresh" and incorrect comment formatting

[How]
Remove variable, reindent comments

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>