drm/amd/display: Constify struct timing_generator_funcs

'struct timing_generator_funcs' are not modified in these drivers.

Constifying these structures moves some data to a read-only section, so
increases overall security, especially when the structure holds some
function pointers.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>