drm/amdgpu: Rename VCN clock gating function for consistency

Change the function name from vcn_v2_5_enable_clock_gating_inst
to vcn_v2_5_enable_clock_gating to ensure consistency in naming.

Fixes the below with gcc W=1:
drivers/gpu/drm/amd/amdgpu/vcn_v2_5.c:781: warning: expecting prototype for vcn_v2_5_enable_clock_gating_inst(). Prototype was for vcn_v2_5_enable_clock_gating() instead

Cc: Leo Liu <leo.liu@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>