drm: Fix framebuffer leak

Do not leak framebuffer if client provided crtc id found invalid.

Signed-off-by: Nikhil Mahale <nmahale@nvidia.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1502250781-5779-1-git-send-email-nmahale@nvidia.com