drm: vblank: Fix vblank timestamp update

drm: vblank: Fix vblank timestamp update

Commit 3fcdcb270936 ("drm/vblank: Switch to bool in_vblank_irq in
get_vblank_timestamp") inverted a condition by mistake that resulted in
vblank timestamps always being 0 on hardware without a vblank counter.
Fix it.

Fixes: 3fcdcb270936 ("drm/vblank: Switch to bool in_vblank_irq in get_vblank_timestamp")
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170629123720.27173-1-laurent.pinchart+renesas@ideasonboard.com