drm/i915: Generalize intel_is_dram_symmetric()

Decouple intel_is_dram_symmetric() from the raw register values
by comparing just the dram_channel_info structs.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190306203551.24592-9-ville.syrjala@linux.intel.com