drm/i915: Check for get_pages instead of shmem (filp)

This behavior of checking for a shmem backed GEM object was introduced here:
commit 4c914c0
Author: Brad Volkin <[email protected]>
Date:   Tue Feb 18 10:15:45 2014 -0800

    drm/i915: Refactor shmem pread setup

It is possible for an object to not be a shmem backed GEM object (for example
userptr objects). An example of how we hit this failure can be found through
copy_batch() in the command parser because we allocate a userptr object for the
batch which contains privileged instructions. Userptr calls
drm_gem_private_object_init() which explicitly sets the filp to none.

NOTE: I manually retyped this from a test machine. So I haven't even compiled
this exact patch.

v2: Use same logic as from a2a4f916c2f (Kristian, Dave Gordon)

Cc: Chris Wilson <[email protected]>
Cc: Kristian Høgsberg <[email protected]>
Cc: Dave Gordon <[email protected]>
Signed-off-by: Ben Widawsky <[email protected]>
Tested-by: Jordan Justen <[email protected]> (v1)
Reviewed-by: Jordan Justen <[email protected]> (v1)
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: Ben Widawsky

drivers/gpu/drm/i915/i915_gem.c

@@ -489,7 +489,7 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
 
 	*needs_clflush = 0;
 
-	if (!obj->base.filp)
+	if (WARN_ON((obj->ops->flags & I915_GEM_OBJECT_HAS_STRUCT_PAGE) == 0))
 		return -EINVAL;
 
 	if (!(obj->base.read_domains & I915_GEM_DOMAIN_CPU)) {