drm/amd/display: Fix a NULL pointer dereference in dcn20_hwseq.c

drm/amd/display: Fix a NULL pointer dereference in dcn20_hwseq.c

[why]
hws->funcs.dccg_init is accessed without checking if it is NULL, which
may lead to a NULL pointer dereference.

[how]
Add a NULL check before calling dccg_init.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>