drm/i915: fbdev restore mode needs to invalidate frontbuffer

This fbdev restore mode was another corner case that was now
calling frontbuffer flip and flush and making we miss
screen updates with PSR enabled.

So let's also add the invalidate hack here while we don't have
a reliable dirty fbdev op.

v2: As pointed by Paulo: removed seg fault risk, used fb_helper
    when possible and put brackets on if.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Testcase: igt/kms_fbcon_fbt/psr
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>