drm/i915: Handle invalid ilk pipe watermarks correctly.

This function returns an int, but when ilk_validate_pipe_wm fails it
returns false, which is 0 (success). As a result invalid watermarks
are applied, while they should have been rejected.

Fix this by returning -EINVAL.

Fixes: ed4a6a7ca853 ("drm/i915: Add two-stage ILK-style watermark programming (v11)")
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456918563-28696-1-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -2345,7 +2345,7 @@ static int ilk_compute_pipe_wm(struct intel_crtc *intel_crtc,
 		pipe_wm->linetime = hsw_compute_linetime_wm(dev, cstate);
 
 	if (!ilk_validate_pipe_wm(dev, pipe_wm))
-		return false;
+		return -EINVAL;
 
 	ilk_compute_wm_reg_maximums(dev, 1, &max);