drm/vblank: Switch drm_driver->get_vblank_timestamp to return a bool

There's really no reason for anything more:
- Calling this while the crtc vblank stuff isn't set up is a driver
  bug. Those places alrready DRM_ERROR.
- Calling this when the crtc is off is either a driver bug (calling
  drm_crtc_handle_vblank at the wrong time) or a core bug (for
  anything else). Again, we DRM_ERROR.
- EINVAL is checked at higher levels already, and if we'd use struct
  drm_crtc * instead of (dev, pipe) it would be real obvious that
  those are again core bugs.

The only valid failure mode is crap hardware that couldn't sample a
useful timestamp, to ask the core to just grab a not-so-accurate
timestamp. Bool is perfectly fine for that.

v2: Also fix up the one caller, I lost that in the shuffling (Jani).

v3: Fixup commit message (Neil).

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Cc: Eric Anholt <eric@anholt.net>
Cc: Rob Clark <robdclark@gmail.com>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170509140329.24114-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/msm/mdp/mdp5/mdp5_kms.c

@@ -592,23 +592,23 @@ static int mdp5_get_scanoutpos(struct drm_device *dev, unsigned int pipe,
 	return ret;
 }
 
-static int mdp5_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe,
-				     int *max_error,
-				     struct timeval *vblank_time,
-				     unsigned flags)
+static bool mdp5_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe,
+				      int *max_error,
+				      struct timeval *vblank_time,
+				      unsigned flags)
 {
 	struct msm_drm_private *priv = dev->dev_private;
 	struct drm_crtc *crtc;
 
 	if (pipe < 0 || pipe >= priv->num_crtcs) {
 		DRM_ERROR("Invalid crtc %d\n", pipe);
-		return -EINVAL;
+		return false;
 	}
 
 	crtc = priv->crtcs[pipe];
 	if (!crtc) {
 		DRM_ERROR("Invalid crtc %d\n", pipe);
-		return -EINVAL;
+		return false;
 	}
 
 	return drm_calc_vbltimestamp_from_scanoutpos(dev, pipe, max_error,