drm/i915: Do not WARN_ON with small framebuffers.

It's perfectly legal to create a fb with stride < 512, and one of
the kms_plane_scaling subtests creates a very small fb.

Downgrade the WARN_ON to a simple check check, and because this
function is potentially called on every atomic update/pageflip,
downgrade the other WARN_ON to a WARN_ON_ONCE, and do the right
thing here.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180116155331.75175-1-maarten.lankhorst@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/intel_fbc.c

@@ -668,11 +668,13 @@ void intel_fbc_cleanup_cfb(struct drm_i915_private *dev_priv)
 static bool stride_is_valid(struct drm_i915_private *dev_priv,
 			    unsigned int stride)
 {
-	/* These should have been caught earlier. */
-	WARN_ON(stride < 512);
-	WARN_ON((stride & (64 - 1)) != 0);
+	/* This should have been caught earlier. */
+	if (WARN_ON_ONCE((stride & (64 - 1)) != 0))
+		return false;
 
 	/* Below are the additional FBC restrictions. */
+	if (stride < 512)
+		return false;
 
 	if (IS_GEN2(dev_priv) || IS_GEN3(dev_priv))
 		return stride == 4096 || stride == 8192;