drm/i915: fix the FBC work allocation failure path

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: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>