drm/amd/display: make function tables const

It is good practice to make global function tables const to avoid
accidental override.

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