drm/amdgpu/gmc8: switch to using amdgpu_gmc_get_vbios_allocations

The new helper centralizes the logic in one place.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>