drm/i915: Put update_state_fb() next to the fb update

update_state_fb() at the end of intel_find_plane_obj() is misleading as
it leads us to believe the update is done for all code path.

A successful call to intel_alloc_plane_obj() will return and
update_state_fb() is then only needed when we share a fb from another
CRTC. Put the update() function there then.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>