drm/amdkfd: remove dead code in kfd_create_vcrat_image_gpu

kfd_create_vcrat_image_gpu itself checks the avail_size at the start.
So the value of avail_size is at least VCRAT_SIZE_FOR_GPU(16384),
minus struct crat_header(40UL) and struct crat_subtype_compute(40UL) it cannot be less than 0.

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>