drm/amd/display: Make function pointer structs const

const to avoid hard-to-find bugs where some function overrides a
function pointer.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>