drm/amd/display: add const qualifiers to watermark params struct

drm/amd/display: add const qualifiers to watermark params struct

[why]
There are few non const input pointer fields. Setting them to const to
prevent future modification of read-only data.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>