drm/i915: Fix wrong return value in intel_atomic_check()

drm/i915: Fix wrong return value in intel_atomic_check()

In the case of calling check_digital_port_conflicts() failed, a
negative error code -EINVAL should be returned.

Fixes: bf5da83e4bd80 ("drm/i915: Move check_digital_port_conflicts() earier")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200802111535.5200-1-tianjia.zhang@linux.alibaba.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>