drm/i915: Constify load detect mode

Make the mode used for load detection const, and adjust all relevant
functions to accept a const mode.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170518193837.393-2-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>