drm/vblank: Tune drm_crtc_accurate_vblank_count() WARN down to a debug

drm/vblank: Tune drm_crtc_accurate_vblank_count() WARN down to a debug

Since commit 632c6e4edef1 ("drm/vblank: Fix flip event vblank count")
even drivers that don't implement accurate vblank timestamps will end
up using drm_crtc_accurate_vblank_count(). That leads to a WARN every
time drm_crtc_arm_vblank_event() gets called. The could be as often
as every frame for each active crtc.

Considering drm_crtc_accurate_vblank_count() is never any worse than
the drm_vblank_count() we used previously, let's just skip the WARN
unless DRM_UT_VBL is enabled. That way people won't be bothered by
this unless they're debugging vblank code. And let's also change it
to WARN_ONCE() so that even when you're debugging vblank code you
won't get drowned by constant WARNs.

Cc: stable@vger.kernel.org
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: "Szyprowski, Marek" <m.szyprowski@samsung.com>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Reported-by: Andrzej Hajda <a.hajda@samsung.com>
Fixes: 632c6e4edef1 ("drm/vblank: Fix flip event vblank count")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171023152540.15364-1-ville.syrjala@linux.intel.com
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>