drm/i915/fbc: Fix nuke for pre-snb platforms

drm/i915/fbc: Fix nuke for pre-snb platforms

The MSG_FBC_REND_STATE register only exists on snb+. For older
platforms (would also work for snb+) we can simply rewite DSPSURF
to trigger a flip nuke.

While generally RMW is considered harmful we'll use it here for
simplicity. And since FBC doesn't exist in i830 we don't have to
worry about the DSPSURF double buffering hardware fails present
on that platform.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702153723.24327-3-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>