drm/amd/display: move dpcd debugfs members setup

Into the function that creates the debugfs files rather
than setting them explicitly in the callers.

Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>