drm/i915: Fix error handling for the NV12 fb dimensions check

Let's not leak obj->framebuffer_references when we decide that
the framebuffer domensions are not suitable for NV12.

Cc: stable@vger.kernel.org
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Vidya Srinivas <vidya.srinivas@intel.com>
Fixes: e44134f2

 ("drm/i915: Add NV12 support to intel_framebuffer_init")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181029140031.11765-1-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>