drm/amd/display: Add allow_clock_gating to dcn42 dccg

drm/amd/display: Add allow_clock_gating to dcn42 dccg

[Why]
The allow_clock_gating function is present in all other DCN versions
and is required to properly migrate DCCG registers access from hwseq
to the dccg component, resolving register conflicts.

[How]
Add the missing .allow_clock_gating function pointer to the
dccg42_funcs struct.

Signed-off-by: Roman Li <roman.li@amd.com>
Acked-by: Chenyu Chen <chen-yu.chen@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>