drm/i915: Propagate error from drm_gem_object_init()

Propagate the real error from drm_gem_object_init(). Note this also
fixes some confusion in the error return from i915_gem_alloc_object...

v2:
(Matthew Auld)
  - updated new users of gem_alloc_object from latest drm-nightly
  - replaced occurrences of IS_ERR_OR_NULL() with IS_ERR()
v3:
(Joonas Lahtinen)
  - fix double "From:" in commit message
  - add goto teardown path
v4:
(Matthew Auld)
  - rebase with i915_gem_alloc_object name change

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461587533-8841-1-git-send-email-matthew.auld@intel.com
[Joonas: Removed spurious " = NULL" from _init() function]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index eb7cb94..d30df876 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -10321,8 +10321,8 @@
 
 	obj = i915_gem_object_create(dev,
 				    intel_framebuffer_size_for_mode(mode, bpp));
-	if (obj == NULL)
-		return ERR_PTR(-ENOMEM);
+	if (IS_ERR(obj))
+		return ERR_CAST(obj);
 
 	mode_cmd.width = mode->hdisplay;
 	mode_cmd.height = mode->vdisplay;