drm/edid: constify struct edid passed around in callbacks and closure

Finalize detailed timing parsing constness by making struct edid also
const in callbacks and closure.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8229be49b5a7686856c17428aa9291b4c4cf1bd5.1648477901.git.jani.nikula@intel.com