Since we use a worker to enable FBC on the CRTC, it is possible for the
CRTC to be switched off before we run. In this case, the CRTC will not
allow us to wait upon a vblank, so remove the DRM_ERROR as this is very
much expected.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102410
Fixes: ca18d51d ("drm/i915/fbc: wait for a vblank instead of 50ms when enabling")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170825150215.19236-1-chris@chris-wilson.co.ukReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>