drm/i915: Allow calling intel_edp_drrs_enable twice

Do not make it an error to call intel_edp_drrs_enable while drrs has
already been enabled, instead exit silently in this case.

This is a preparation patch for ensuring that DRRS is enabled on fastsets.

Note that the removed WARN_ON could also be triggered from userspace
through the i915_drrs_ctl debugfs entry which was added by
commit 35954e88 ("drm/i915: Runtime disable for eDP DRRS")

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181220132120.15318-2-hdegoede@redhat.com