drm/amdgpu: rename nv_is_headless_sku()

for headless NAVI ASICs

Signed-off-by: Flora Cui <flora.cui@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>