drm/i915: Handle invalid ilk pipe watermarks correctly.

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: 109c35f21ff5 ("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>