drm/i915: Fix possible null dereference in framebuffer_info debugfs function

Found by static code analysis tool.

v2: Inserted block instead of goto & renamed variables (Chris)
v3: Aligned code as per the opening brace (Chris)
    Rebased on top of nightly (Daniel)

Signed-off-by: Namrta Salonie <namrta.salonie@intel.com>
Signed-off-by: Deepak S <deepak.s@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>