drm/edid: constify struct detailed_timing in lower level parsing

Start constifying the struct detailed_timing pointers being passed
around from bottom up.

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/0b7fafcc7784db0003e454544916c273a9eb1250.1648477901.git.jani.nikula@intel.com