drm/i915: remove redundant fbc warnings

The fbc enabled/active sanity checks are already done in
__intel_fbc_disable so no need to do them again.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467710914-15146-1-git-send-email-matthew.auld@intel.com