drm/amdgpu: rename gmc_v8_0_init_compute_vmid

It should be gfx_v8_0_init_compute_vmid since it's
part of the gfx block.

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