drm: Fix return value of drm_framebuffer_init()

In its original version, drm_framebuffer_init() returned a negative int
if drm_mode_object_get() failed (f453ba04, "DRM: add mode setting
support").

This was accidentally disabled by commit 4b096ac1 ("drm: revamp
locking around fb creation/destruction"). Thus, drm_framebuffer_init()
pretends success if drm_mode_object_get() failed.

Reinstate the original behaviour. Also fix erroneous kernel-doc of
drm_mode_object_get().

Fixes: 4b096ac1 ("drm: revamp locking around fb creation/
    destruction")
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>