drm/i915: Suppress spurious underruns on gen2

Often we seem to detect an underrun right after modeset on gen2.
It seems to be a spurious detection (potentially the pipe is still
in a wonky state when we enable the planes). An extra vblank wait
seems to cure it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429101034.8208-13-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>