drm/i915: Drop fb reference on load_detect_pipe failure path

drm/i915: Drop fb reference on load_detect_pipe failure path

When intel_modeset_setup_plane_state() fails drop the local framebuffer
reference before jumping to the error, otherwise we leak the framebuffer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: edde361711ef ("drm/i915: Use atomic state to obtain load detection crtc, v3.")
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171207220025.22698-1-chris@chris-wilson.co.uk
(cherry picked from commit 3e72be177cf19ab3d62b3084d424dce7e71d847f)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>