drm/i915: make dbuf configurations const

Ensure const data goes to rodata.

Fixes: ff2cd863 ("drm/i915: Correctly map DBUF slices to pipes")
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200219154542.19574-1-jani.nikula@intel.com