drm/amd/amdgpu: Tidy up gfx_v9_0_enable_gfx_cg_power_gating()

Make it consistent in style with the other CG/PG enable functions...

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>