fill_page_dma
fill_page_dma(px_base(pd), vm->scratch[vm->top]->encode, count);
fill_page_dma(struct drm_i915_gem_object *p, const u64 val, unsigned int count);
#define fill_px(px, v) fill_page_dma(px_base(px), (v), PAGE_SIZE / sizeof(u64))