drm/i915: Fix double unref in intelfb_alloc failure path

In intelfb_alloc(), if the call to intel_pin_and_fence_fb_obj() fails,
the bo is unrefed twice: By drm_framebuffer_remove() and once more by
drm_gem_object_unreference(). Fix it.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/cd7b33330621a350b0159ec5e098297b139cfaf7.1446892879.git.lukas@wunner.de


Signed-off-by: Jani Nikula <jani.nikula@intel.com>