drm/amd/display: Fix description of module parameter dc_log

It was incorrectly referencing the dc parameter, resulting in an empty
description of the dc_log parameter.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>