Always update the currrent crtc, fb and vertical offset after calling
enable_fbc. We were forgetting to do so along the failure paths when
enabling fbc synchronously. Fix this with a new helper to enable_fbc()
and update the state simultaneously.

v2: Improve commit message (Chris).
v3: Constify struct drm_framebuffer (Ville).
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>