drm/i915: Check input parameter for NULL

[why]
i915_gem_object_put_pages_phys() frees pages and standard
pattern is to allow caller to not care if it's
NULL or not. This will reduce burden on
the callers to perform this check.

[how]
Fix it by adding Null check.

Signed-off-by: Yongzhi Liu <lyz_cs@pku.edu.cn>

drivers/gpu/drm/i915/gem/i915_gem_phys.c

@@ -95,6 +95,8 @@ void
 i915_gem_object_put_pages_phys(struct drm_i915_gem_object *obj,
 			       struct sg_table *pages)
 {
+	if (!pages)
+		return;
 	dma_addr_t dma = sg_dma_address(pages->sgl);
 	void *vaddr = sg_page(pages->sgl);