drm/i915: Set hwmode during readout.

This was introduced after converting hw readout to atomic,
so it should have been part of the revert too.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90929


Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>