drm/edid: use macros for ELD offsets and values

We have the macros, use them. No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/36f00f018280a04d2db806574d337f7512986c86.1509545641.git.jani.nikula@intel.com