DocBook/drm: Use a <variablelist> for vblank ioctls

    <title>VBlank event handling</title>

    <para>

      The DRM core exposes two vertical blank related ioctls:

      DRM_IOCTL_WAIT_VBLANK and DRM_IOCTL_MODESET_CTL.

<!--!Edrivers/char/drm/drm_irq.c-->

    </para>

      <variablelist>

        <varlistentry>

          <term>DRM_IOCTL_WAIT_VBLANK</term>

          <listitem>

            <para>

      DRM_IOCTL_WAIT_VBLANK takes a struct drm_wait_vblank structure

      as its argument, and is used to block or request a signal when a

      specified vblank event occurs.

              This takes a struct drm_wait_vblank structure as its argument,

              and it is used to block or request a signal when a specified

              vblank event occurs.

            </para>

          </listitem>

        </varlistentry>

        <varlistentry>

          <term>DRM_IOCTL_MODESET_CTL</term>

          <listitem>

            <para>

      DRM_IOCTL_MODESET_CTL should be called by application level

      drivers before and after mode setting, since on many devices, the

      vertical blank counter is reset at that time.  Internally,

      the DRM snapshots the last vblank count when the ioctl is called

      with the _DRM_PRE_MODESET command so that the counter won't go

      backwards (which is dealt with when _DRM_POST_MODESET is used).

              This should be called by application level drivers before and

              after mode setting, since on many devices the vertical blank

              counter is reset at that time.  Internally, the DRM snapshots

              the last vblank count when the ioctl is called with the

              _DRM_PRE_MODESET command so that the counter won't go backwards

              (which is dealt with when _DRM_POST_MODESET is used).

            </para>

          </listitem>

        </varlistentry>

      </variablelist>

<!--!Edrivers/char/drm/drm_irq.c-->

    </para>

    <para>

      To support the functions above, the DRM core provides several